ccna60d/d36-EIGRP.md
2020-12-29 17:37:48 +08:00

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# 第36天 增强的内部网关路由协议
**Enhanced Interior Gateway Routing Protocol, EIGRP**
___
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## 第36天任务
- 阅读今天的课文
- 复习昨天的课文
- 完成今天的实验
- 阅读ICND2记诵指南
增强的内部网关路由协议是一种由思科开发的、专有内部网关协议a proprietary Interior Gateway Protocol(IGP) that was developed by Cisco。EIGRP包含了一些传统的距离矢量特性比如水平拆分split horizon还包含了与那些被链路状态路由协议所用到的类似特性比如增量更新EIGRP includes traditional Distance Vector characteristics, such as split horizon, as well as characteristics that are similar to those used by Link State routing protocols, such as incremental updates
尽管有着链路状态路由协议的某些特性但EIGRP是被分类到距离矢量路由协议类别中的被指为**一种高级的距离矢量路由协议**an advanced Distance Vector routing protocol。EIGRP**直接在IP上运行使用协议编号88**。
今天将学习以下内容:
- 思科公司EIGRP概述与基础知识, Cisco EIGRP overview and fundamentals
- EIGRP配置基础, EIGRP configuration fundamentals
- EIGRP的各种报文, EIGRP messages
- EIGRP的邻居发现与邻居维护, EIGRP neighbour discovery and maintenance
- 各种度量值、弥散更新算法DUAL与拓扑表Metrics, DUAL, and the topology table
- 相等与不相等开销下的负载均衡equal cost and unequal cost load sharing
- 采用EIGRP作为默认路由default routing using EIGRP
- EIGRP网络中的水平分割, split horizon in EIGRP networks
- EIGRP的存根路由EIGRP stub routing
- EIGRP的路由汇总EIGRP route summarisation
- 掌握被动接口understanding passive interfaces
- 掌握EIGRP路由器ID的用法understanding the use of the EIGRP router ID
- EIGRP的日志与报表EIGRP logging and reporting
本课程对应了以下CCNA大纲要求
- 配置并验证EIGRP单一自治系统configure and verify EIGRP(single AS)
- 可行距离/可行的后续路由/报告的距离/通告的距离分别是什么Feasible Distance/Feasible Successor routes/Reported Distance/Advertised Distance
- 可行性条件Feasiblity condition
- 度量值综合Metric composition
- 路由器IDRouter ID
- 自动汇总Auto summary
- 路径选择path selection
+ EIGRP的负载均衡load balancing
- 开销一样时
- 开销不同时
- 什么是EIGRP的被动接口passive interfaces
## 思科EIGRP概述与基础知识
为解决其先前的专有距离矢量路由协议 -- 内部网关路由协议Interior Gateway Routing Protocol, IGRP的某些缺陷思科公司开发了EIGRP。相比路由信息协议Routing Information Protocol, RIPIGRP确实有着一些改进比如对更多跳数的支持但仍然有着那些传统距离矢量路由协议的局限这些局限如下所示
- 发送完整的周期性路由更新sending full periodic routing updates
- 跳数限制a hop limitation
- 缺少对变长子网掩码的支持the lack of VLSM support
- 收敛速度慢slow convergence
- 缺少防止环回形成的机制the lack of loop prevention mechanisms
与传统距离矢量路由协议往邻居发送包含所有路由信息的周期性路由更新不同EIGRP发送的是非周期性的、增量式路由更新以将路由信息在整个路由域中分发unlike the traditional Distance Vector routing protocols, which send their neighbours periodic routing updates that contain all routing information, EIGRP sends non-periodic incremental routing updates to ditribute routing information throughout the routing domain。只有在网络拓扑发生变化时才会发送EIGRP增量更新。
默认下的RIP一种以前的CCNA考试项目有着15的跳数限制这就令到RIP只适合于小型网络。EIGRP默认跳数限制为100; 但此数值可被管理员在配置EIGRP时使用路由器配置命令`metric maximum-hops <1-255>`, 予以手动调整。这就令到EIGRP具备对有着多达数百台路由器网络的支持能力使其具备了更大的可伸缩性从而对较大型网络也是适合的。
增强的IGRP采用了两个独特的**类型/长度/数值三联体数据结构**来表示和传输路由条目Enhanced IGRP uses two unique Type/Length/Value(TLV) triplets to carry route entries。这两个TLVs分别是**内部EIGRP路由TLV**与**外部EIGRP路由TLV**the Internal EIGRP Route TLV and the External EIGRP Route TLV 分别用于内部及外部的EIGRP路由。两种TLVs都包含了一个8位的前缀长度字段an 8-bit Prefix Length field, 用于指明用于目的网络子网掩码的位数。包含在此字段中的该信息就令到EIGRP能够支持不同的子网划分了。
增强的IGRP比起传统的距离矢量路由协议收敛得快得多。除了仅仅依赖于计时器EIGRP还使用其拓扑表中的信息来找出那些替代路径。EIGRP也可以在未能于本地路由器的拓扑表中找出替代路径的情况下向邻居路由器查询信息。本课程模块后面会讲到EIGRP的拓扑表。
而为了确保整个网络中没有环回路径EIGRP使用了**弥散更新算法**Diffusing Update Algorithm, DUAL使用此算法来对邻居通告的所有路由进行追踪并随后选出到目的网络最优的无环回路径。弥散更新算法是EIGRP的一个核心概念将在本课程模块的稍后讲到。
## EIGRP配置基础
**EIGRP Configuration Fundamentals**
在思科IOS软件中是通过使用全局配置命令`router eigrp [ASN]`来开启增强的IGRP的。关键字`[ASN]`指定EIGRP的自治系统编号autonomous system number, ASN这是一个32位整数大小介于1-65535之间。除了本章后面将涉及的其它因素之外**运行EIGRP的那些路由器都必须位处同一自治系统中**,以成功形成邻居关系。在全局配置命令`router eigrp [ASN]`之后, 路由器就转变为EIGRP路由器配置模式EIGRP Router Configuration mode在这里就可以对那些与EIGRP有关的参数进行配置了。所配置的ASN可在命令`show ip protocols`的输出中进行验证,如下面所示:
```console
R1#show ip protocols
Routing Protocol is “eigrp 150”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
...
[Truncated Output]
```
而除了`show ip protocols`命令,命令`show ip eigrp neighbours`会打印出所有EIGRP邻居以及这些邻居各自自治系统的信息。该命令及其可用选项将在本课程模块的后面进行详细讲解。在那些运行了多个EIGRP实例的路由器上可使用`show ip eigrp [ASN]`命令,来查看只与在此命令中所指定的自治系统有关的信息。下面的输出演示了这个命令的使用:
```console
R1#show ip eigrp 150 ?
interfaces IP-EIGRP interfaces
neighbors IP-EIGRP neighbors
topology IP-EIGRP topology table
traffic IP-EIGRP traffic statistics
```
在上面的输出中150就是自治系统编号ASN。如`show ip eigrp`命令没有指定自治系统那么在思科IOS软件中该命令默认将打印出所有EIGRP实例的信息。
而一旦处于路由器配置模式Router Configuration mode就要使用`network`命令来指明要在哪些网络接口上开启EIGRP路由了once in Router Configuration mode, the `network` command is used to specify the network(s) (interfaces) for which EIGRP routing will be enabled。在使用`network`命令并指明了一个大的有类网络后该启用了EIGRP的路由器将完成以下动作
- 对位处该指明的有类网络范围里的网络开启EIGRPEIGRP is enabled for networks that fall within the specified classful network range.
- 以这些直连子网生成EIGRP的拓扑表the topology table is populated with these directly connected subnets.
- 从这些与子网关联的接口发出EIGRP Hello 数据包EIGRP Hello packets are sent out of the interfaces associated with these subnets.
- EIGRP将这些网络经由更新报文通告给EIGRP邻居EIGRP advertises the network(s) to EIGRP neighbours in Update messages.
- 在报文交换的基础上那些EIGRP路由此时就被加入到路由器的IP路由表中Based on the exchange of messages, EIGRP routes are then added to the IP routing table.
比如假设某台路由器上配置了以下这些环回接口:
- `Loopback0` -- IP地址 `10.0.0.0/24`
- `Loopback1` -- IP地址 `10.0.0.1/24`
- `Loopback2` -- IP地址 `10.0.0.2/24`
- `Loopback3` -- IP地址 `10.0.0.3/24`
如EIGRP已开启使用且将路由器配置命令`network`与大的有类`10.0.0.0/8`网络一道进行了使用,同时**所有4个环回接口**all four Loopback interfaces又都开启了EIGRP路由的话那么下面就给出了此种情况下`show ip eigrp interfaces`的输出演示:
```console
R1#show ip eigrp interfaces
IP-EIGRP interfaces for process 150
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Lo0 0 0/0 0 0/10 0 0
Lo1 0 0/0 0 0/10 0 0
Lo2 0 0/0 0 0/10 0 0
Lo3 0 0/0 0 0/10 0 0
```
可使用`show ip protocols`命令,来对大的有类`10.0.0.0/8`网络上EIGRP的启用情况进行验证。此命令的输出如下所示
```console
R1#show ip protocols
Routing Protocol is “eigrp 150”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 150
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Maximum path: 4
Routing for Networks:
10.0.0.0
Routing Information Sources:
Gateway Distance Last Update
Distance: internal 90 external 170
```
使用命令`show ip eigrp topology`可查看到EIGRP的拓扑表。此命令的输出如下所示
```console
R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(150)/ID(10.3.3.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.3.3.0/24, 1 successors, FD is 128256
via Connected, Loopback3
P 10.2.2.0/24, 1 successors, FD is 128256
via Connected, Loopback2
P 10.1.1.0/24, 1 successors, FD is 128256
via Connected, Loopback1
P 10.0.0.0/24, 1 successors, FD is 128256
via Connected, Loopback0
```
> **注意:** 本课程模块稍后会对拓扑表、EIGRP的Hello数据包及更新数据包进行详细讲解。本小节仅着重于EIGRP的配置实施EIGRP configuration implementation
使用`network`命令来指明一个大的有类网络a major classful network就令到位于该有类网络中的多个子网得以在最小配置下同时被通告出去。但可能存在管理员不想对某个有类网络中的所有子网都开启EIGRP路由的情形。比如参考前一示例中`R1`上所配置的环回接口,假设只打算对`10.1.1.0/24`及`10.3.3.0/24`子网开启EIGRP路由而不愿在`10.0.0.0/24`及`10.2.2.0/24`开启EIGRP路由。那么很明显这在使用`network`命令时,对这些网络(也就是`10.1.1.0`及`10.3.3.0`予以指明就可以做到思科IOS软件仍会将这些语句转换成大的有类`10.0.0.0/8`网络,如下所示:
```console
R1(config)#router eigrp 150
R1(config-router)#network 10.1.1.0
R1(config-router)#network 10.3.3.0
R1(config-router)#exit
```
尽管有着上面的配置,但`show ip protocols`命令给出的确实下面的输出:
```console
R1#show ip protocols
Routing Protocol is “eigrp 150”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 150
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Maximum path: 4
Routing for Networks:
10.0.0.0
Routing Information Sources:
Gateway Distance Last Update
Distance: internal 90 external 170
```
> **注意:** 一个常见的误解就是关闭EIGRP的自动汇总特性就能解决此问题但是这与`auto-summary`命令一点关系都没有。比如,假设对在前一示例中的配置执行了`no auto-summary`命令,如下所示:
```console
R1(config)#router eigrp 150
R1(config-router)#network 10.1.1.0
R1(config-router)#network 10.3.3.0
R1(config-router)#no auto-summary
R1(config-router)#exit
```
`show ip protocols`命令仍将显示对网络`10.0.0.0/8`开启了EIGRP如下面的输出所示
```console
R1#show ip protocols
Routing Protocol is “eigrp 150”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 150
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is not in effect
Maximum path: 4
Routing for Networks:
10.0.0.0
Routing Information Sources:
Gateway Distance Last Update
Distance: internal 90 external 170
```
为了提供到**对那些开启EIGRP路由的网络进行更细粒度的控制**思科IOS软件支持在对EIGRP进行配置时将**通配符掩码**与`network`语句一起配合使用in order to provide more granular control of the networks that are enabled for EIGRP routing, Cisco IOS software supports the use of wildcard masks in conjunction with the `network` statement when configuring EIGRP。这里的通配符掩码以与ACLs中用到的通配符掩码类似的方式运作而与网络的子网掩码是不相干的。
作为一个示例,命令`network 10.1.1.0 0.0.0.255`将匹配到网络`10.1.1.0/24`、`10.1.1.0/26`及`10.1.1.0/30`网络。参考上一输出中所配置的那些环回借口the Loopback interfaces为将`R1`配置为对`10.1.1.0/24`及`10.3.3.0/24`子网开启EIGRP路由且不对`10.0.0.0/24`子网或`10.2.2.0`子网开启,就应将其如下面那样进行配置:
```console
R1(config)#router eigrp 150
R1(config-router)#network 10.1.1.0 0.0.0.255
R1(config-router)#network 10.3.3.0 0.0.0.255
R1(config-router)#exit
```
使用命令`show ip protocols`,就可对此配置进行验证,如下所示:
```console
R1#show ip protocols
Routing Protocol is “eigrp 150”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 150
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Maximum path: 4
Routing for Networks:
10.1.1.0/24
10.3.3.0/24
Routing Information Sources:
Gateway Distance Last Update
Distance: internal 90 external 170
```
此外,还可以使用命令`show ip eigrp interfaces`,确认到仅已对`Loopback1`与`Loopback3`开启了EIGRP路由
```console
R1#show ip eigrp interfaces
IP-EIGRP interfaces for process 150
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Lo1 0 0/0 0 0/10 0 0
Lo3 0 0/0 0 0/10 0 0
```
如上面所示,因为这里的通配符掩码配置,而仅在`Loopback1`和`Loopback3`上启用了EIGRP路由。
这里重要的是记住除了使用通配符掩码,**也可以使用子网掩码来配置`network`命令**。在此情况下思科IOS软件将翻转子网掩码而使用通配符掩码来保存该命令。比如参照路由器上同样的环回借口路由器`R1`也可被如下这样进行配置:
```console
R1(config-router)#router eigrp 150
R1(config-router)#network 10.1.1.0 255.255.255.0
R1(config-router)#network 10.3.3.0 255.255.255.0
R1(config-router)#exit
```
基于此种配置就在运行配置中输入了下面的参数这里使用了管道pipe取得运行配置中感兴趣的部分
```console
R1#show running-config | begin router eigrp
router eigrp 150
network 10.1.1.0 0.0.0.255
network 10.3.3.0 0.0.0.255
auto-summary
```
通过上面的配置可以看出,可与那些`show`命令一道运用管道,来获得更细的粒度。这对于那些有着编程知识的人来说,是一种熟悉的概念。
如将某个网络上的特定地址与通配符一起使用那么思科IOS软件将执行一次逻辑与运算a logical AND operation, 从而确定出那个要启用EIGRP的网络。比如在执行了`network 10.1.1.15 0.0.0.255`命令时思科IOS软件会执行以下动作
- 将通配符掩码翻转为子网掩码值`255.255.255.0`
- 执行一次逻辑与操作
- 将命令`network 10.1.1.0 0.0.0.255`加入到配置中
本示例中所用到的`network`配置,如下面输出所示:
```console
R1(config)#router eigrp 150
R1(config-router)#network 10.1.1.15 0.0.0.255
R1(config-router)#exit
```
那么基于此配置,路由器上的运行配置,就会显示如下内容:
```console
R1#show running-config | begin router eigrp
router eigrp 150
network 10.1.1.0 0.0.0.255
auto-summary
```
如上面的配置所示不管是使用通配符子网掩码还是子网掩码都会在思科IOS软件中造成同样的操作并得到同样的`network`语句配置。
> **真实世界的部署**
> 当在生产网络中对EIGRP进行配置时**一般做法都是使用全0的通配符掩码或全1的子网掩码**。比如,`network 10.1.1.1 0.0.0.0`及`network 10.1.1.1 255.255.255.255`两个命令都会执行同样的动作。全0的通配符掩码或全1的子网掩码的使用就将思科IOS软件配置为与一个具体接口地址进行匹配而不考虑在接口本身上所配置哦子网掩码了。这两个命令都会匹配到配置了比如`10.1.1.1/8`、`10.1.1.1/16`、`10.1.1.1/24`, 以及`10.1.1.1/30`等地址的接口。这些命令的用法如下面的输出所示:
```console
R1(config)#router eigrp 150
R1(config-router)#network 10.0.0.1 0.0.0.0
R1(config-router)#network 10.1.1.1 255.255.255.255
R1(config-router)#exit
```
`show ip protocols`命令将验证到路由器对于两个`network`语句,都是以相似的方式进行处理的,如下所示:
```console
R1#show ip protocols
Routing Protocol is “eigrp 150”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 150
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Maximum path: 4
Routing for Networks:
10.0.0.1/32
10.1.1.1/32
Routing Information Sources:
Gateway Distance Last Update
Distance: internal 90 external 170
```
在使用了全1的子网掩码或全1的通配符掩码时就会在所指定的匹配的接口上开启EIGRP同时将通告那个接口所位处的网络。也就是说EIGRP不会通告上面输出中的`/32`地址而是通告基于配置在匹配接口上的子网掩码的具体网络。此配置的用法是独立于配置所匹配具体接口子网掩码的when a subnet mask with all ones or a wildcard mask with all zeros is used, EIGRP is enabled for the specified(matched) interface and the network the interface resides on is advertised. In other words, EIGRP will not advertise the /32 address in the above but, instead, the actual network based on the subnet mask configured on the matched interface. The use of this configuration is independent of the subnet mask configuration on the actual interface matched
## EIGRP的各种报文
**EIGRP Messages**
本小节将对EIGRP所用到的各种类型的报文进行说明。但是在深入到各种不同报文类型前首要的是对EIGRP数据包头部有扎实的掌握这正是包含这些报文的地方。
### EIGRP数据包头部
**EIGRP Packet Header**
尽管对EIGRP数据包格式的具体了解是超出CCNA考试要求的但对EIGRP数据包头部的扎实掌握对于完整理解EIGRP这种路由协议的整体运作原理是很重要的。下图36.1对EIGRP数据包头部进行了演示
![EIGRP数据包头部的各种字段](images/3601.png)
*图 36.1 -- EIGRP数据包头部的各种字段*
在EIGRP数据包头部其中的4位版本字段the 4-bit Version field用于表明该协议的版本。当前的思科IOS镜像都支持EIGRP版本`1.x`。后面的4为OPcode字段the 4-bit OPCode field则指定了EIGRP数据包或报文的类型。不同EIGRP数据包类型都被分配了一个唯一的OPcode数值以将其与其它数据包类型进行区分。本课程模块后面会对这些报文类型进行说明。
而那个24位的校验和字段the 24-bit Checksum field, 是用于对该EIGRP数据包做完整性检查的a sanity check。该字段基于完整的EIGRP数据包的, 是排除了IP头部的。32位的标志字段the 32-bit Flags field用于表明该EIGRP数据包或报文是一个新EIGRP邻居的`INIT`, 还是EIGRP的可靠传输协议Reliable Transport Protocol, RTP条件接收下the Conditional Receive, CR的`INIT`。这里的RTP及CR都将在本课程模块稍后讲到。
接着便是32位的序列字段the 32-bit Sequence field其指明了被EIGRP可靠传输协议所用到的顺序编号the sequence number用于可靠数据包的顺序投送。而32位的确认字段the 32-bit Acknowledgment field则被用于EIGRP可靠数据包的接收确认。
后面的32位自治系统编号字段the 32-bit Autonomous System Number field, 指定了该EIGRP域the EIGRP domain的自治系统编号ASN。最后的32位类型/长度/值三联体字段就被用于路由条目运送及EIGRP弥散更新算法信息的提供了Finally, the 32-bit Type/Length/Value(TLV) triplet field is used to carry route entries and provides EIGRP DUAL information。EIGRP支持几种不同类型的TLVs最常用的是下面几种
- 参数TLV, 有着建立邻居关系的那些参数the Parameters TLV, which has the parameters to establish neighbour relationships
- 序号TLV为RTP所用到的TLVthe Sequence TLV, which is used by RTP
- 下一次多播序号TLVRTP使用的TLVthe Next Multicast Sequence TLV, which is used by RTP
- EIGRP内部路由TLV用于内部EIGRP路由the EIGRP Internal Route TLV, Which is used for internal EIGRP routes
- EIGRP外部路由TLV用于外部的EIGRP路由the EIGRP External Route TLV, which is used for external EIGRP routes
> **注意:** 并不要求对EIGRP的各种TLVs有详细了解。
下图36.2演示了一个抓包到的EIGRP数据包的所呈现的不同字段
![EIGRP数据包头部的抓包](images/3602.png)
*图 36.2 -- 对EIGRP数据包头部的抓包*
在该EIGRP数据包头部4位的OPCode字段被用于指明该EIGRP数据包类型或报文。EIGRP使用到不同的报文或数据包类型它们是**Hello数据包**Hello packets、**确认数据包**Acknowledgment packets、**更新数据包**Update packets、**查询数据包**Query packets、**应答数据包**Reply packets以及**请求数据包**Request packets共计6种报文或数据包类型。将在随后的小节对这些类型的数据包进行说明。
### Hello数据包
**Hello Packets**
在某台路由器上对某个特定网络开启了增强的IGRP后其就会发送Hello数据包Enhanced IGRP sends Hello packets once it has been enabled on a router for a particular network。这些报文被用于邻居的识别同时邻居一经识别后Hello报文就用于在邻居间作为一种保持活动机制发挥作用these messages are used to identify neighbours and, once identified, serve or function as a keepalive mechanism between neighbours。**EIGRP的邻居发现与维护机制**,将在本课程模块的后面进行说明。
EIGRP的Hello数据包是发送到**链路本地多播组地址the Link Local Multicast group address`224.0.0.10`**上的。由EIGRP发出的Hello数据包是**不需要发出确认数据包来确认其已收到**的Hello packets sent by EIGRP do not require an Acknowledgment to be sent confirming that they were received。因为Hello数据包不需要显式的确认所以它们被分类为**不可靠的EIGRP数据包**Hello packets are classified as unreliable EIGRP packets。EIGRP Hello数据包的**OPCode为5**。
### 确认数据包
**Acknowledgment Packets**
EIGRP确认数据包就是一个**不包含数据的**EIGRP Hello数据包。EIGRP使用确认数据包来对EIGRP数据包的可靠送达进行确认。这些确认数据包the ACK packets总是发送到一个单播地址a Unicast address, 该地址就是可靠数据包发送方的源地址the source address of the sender of the reliable packet而并不是EIGRP的多播组地址了。此外确认数据包将总是会包含一个非零的确认编号a non-zero acknowledgment number。确认数据包使用了Hello数据包相同的OPCode, 因为其本来就是一个不包含任何信息的Hello数据包。其OPCode为5。
### 更新数据包
**Update Packets**
增强IGRP的更新数据包被用于传送目标的可达性used to convey reachability of destinations。也就是说更新数据包包含了EIGRP的路由更新。在发现了一个新的邻居时就会通过单播发出更新数据包往该新的邻居如此新的邻居就能够建立起自己的EIGRP拓扑表了。在其它情况比如某链路的开销改变时就会经由多播发出更新数据包。重要的是记住**更新数据包都是可靠地传输的,且总是要求显式的确认**。分配给**更新数据包的OPCode是1**。下图36.3演示了一个EIGRP的更新数据包
![EIGRP的更新数据包](images/3603.png)
*图 36.3 -- EIGRP的更新数据包*
> **注意:** 并不要求对EIGRP各种数据包中的所包含的信息有深入了解。
### 查询数据包
**Query Packets**
增强IGRP的查询数据包是多播的并被用于请求可靠的路由信息。EIGRP的查询数据包是在某条路由不可用但该路由器却需要为快速收敛而需要就该路由的状态进行询问时所发送给其邻居的数据包。如发出查询数据包的路由器未能从其某些邻居收到响应那么其就会再度向那些未响应的邻居发出一次查询。如在16次尝试后都没有响应那么该EIGRP邻居关系将被重置。本课程模块后面将对此概念进行更为深入的说明。分配给EIGRP**查询数据包的OPCode为3**。
### 应答数据包
**Reply Packets**
增强IGRP的应答数据包是作为对查询数据包的响应发送的。应答数据包用于可靠地响应某个查询数据包。应答数据包是到查询发起方的单播数据包。分配给EIGRP**应答数据包的OPCode为4**。
### 请求数据包
**Request Packets**
增强IGRP的请求数据包**用于从一个或多个邻居处获取特定信息,且是在路由服务器应用中用到的**used in route server applications。这些数据包既**可通过单播、也可通过多播进行发送,但它们总是以不可靠方式传输**。也就是说,它们**无需显式确认**。
> **注意:** 尽管这里的Hello数据包和确认数据包是作为两种独立的数据包类型的但重要的是记住在某些课本中EIGRP的Hello数据包与确认数据包被认为是同一中类型的数据包。这是因为正如在本小节中指出的那样确认数据包就是不包含数据的Hello数据包。
命令`debug eigrp packets`可用于打印出本小节中所讲到的各种不同EIGRP数据包的实时调试信息。要知道此命令还包括了一些这里并没有说到的其它数据包因为这些其它类型数据包超出了当前CCNA考试要求。下面的输出对此命令的用法进行了演示
```console
R1#debug eigrp packets ?
SIAquery EIGRP SIA-Query packets
SIAreply EIGRP SIA-Reply packets
ack EIGRP ack packets
hello EIGRP hello packets
ipxsap EIGRP ipxsap packets
probe EIGRP probe packets
query EIGRP query packets
reply EIGRP reply packets
request EIGRP request packets
retry EIGRP retransmissions
stub EIGRP stub packets
terse Display all EIGRP packets except Hellos
update EIGRP update packets
verbose Display all EIGRP packets
<cr>
```
而`show ip eigrp traffic`命令,则是**用于对本地路由器所发送及接收到的EIGRP数据包的数量进行查看的命令**。该命令同时**还是一个强大的故障排除工具**。比如假设某路由器在发出Hello数据包却并未收到任何回复这可能表明其尚未配置好预期的邻居或者甚至有可能某个确认数据包阻塞了EIGRP数据包For example, if the router is sending out Hello packets but is not receiving any back, this could indicate that the intended neighbour is not configured, or even that an ACK may be blocking EIGRP packets。下面的输出对此命令进行了演示
```console
R2#show ip eigrp traffic
IP-EIGRP Traffic Statistics for AS 150
Hellos sent/received: 21918/21922
Updates sent/received: 10/6
Queries sent/received: 1/0
Replies sent/received: 0/1
Acks sent/received: 6/10
SIA-Queries sent/received: 0/0
SIA-Replies sent/received: 0/0
Hello Process ID: 178
PDM Process ID: 154
IP Socket queue: 0/2000/2/0 (current/max/highest/drops)
Eigrp input queue: 0/2000/2/0 (current/max/highest/drops)
```
下表36.1对本小节中讲到的这些EIGRP的数据包进行了总结以及各自是否以可靠或不可靠方式进行发送
*表 36.1 -- EIGRP数据包总结*
| 报文类型 | 说明 | 发送方式 |
| :------ | :------ | :------ |
| Hello | 用于邻居发现、维护及保持存活 | 不可靠 |
| 确认数据包Acknowledgment | 用于对信息接收的确认 | 不可靠 |
| 更新数据包Update | 用于传达路由信息 | 可靠的 |
| 查询数据包Query | 用于请求指定的路由信息 | 可靠的 |
| 应答数据包Reply | 用于对查询数据包的响应 | 可靠的 |
| 请求数据包Request | 用于路由服务器应用中的信息请求 | 不可靠 |
## EIGRP的邻居发现与邻居维护
**EIGRP Neighbour Discovery and Maintenance**
可将增强的IGRP配置为动态地发现相邻路由器这是默认选项discover neighbouring routers dynamically(default)或者经由管理员手动配置来发现相邻路由器。下面的小节将对这两种方式以及其它有关EIGRP邻居相关话题进行讨论。
### 动态的邻居发现
**Dynamic Neighbour Discovery**
动态邻居发现是通过往目的多播组地址the destination Multicast group address`224.0.0.10`, 发送EIGRP的Hello数据包完成的。动态邻居发现又是紧跟着在路由器配置EIGRP时`network`命令的执行而完成的。此外如早先指出的那样EIGRP数据包直接透过IP、使用协议编号88发送。下图36.4对此基本的EIGRP邻居发现与路由交换过程进行了演示
![EIGRP的邻居发现与路由交换](images/3604.png)
*图 36.4 -- EIGRP的邻居发现与路由交换EIGRP Neighbour Discovery and Route Exchange*
参考图36.4, 在初始化时这些EIGRP邻居便发出Hello数据包以发现其它邻居Referencing Figure 36.4, upon initialisation, the EIGRP neighbours send Hello packets to discover other neighbours。随后邻居们就通过完整更新就其整个路由表进行交换。这些更新包含了所有已知路由信息。因为更新包是可靠发送的接收方就必须对其进行显式确认。
在邻居们完成它们的路由信息交换后,还**将持续交换Hello数据包以维护邻居关系**。此外这些EIGRP邻居路由器以后就将仅发送增量更新了通过增量更新来将其状态或路由变化通告给它们的邻居们。它们再也不会发送完整的更新给邻居们了。
这里重要的是要明白仅简单地在两台或多台路由器上开启EIGRP并不能确保邻居关系的建立。而是还需一些参数必须要匹配这样这些路由器才能成为邻居。**在以下几种情况下就不能建立EIGRP邻居关系**
- EIGRP认证参数不匹配在有配置时Mismatched EIGRP authentication parameters(if configured)
- EIGRP的那些K值不一致Mismatched EIGRP K values
- EIGRP自治系统编号不一致Mismatched EIGRP autonomous system number
- 在EIGRP邻居关系中使用了接口的第二地址Using secondary addresses for EIGRP neighbour relationships
- 邻居并不位于同一子网中the neighbours are not on a common subnet
尽管`show ip eigrp neighbours`命令在动态与静态配置的邻居间没有区别,但**`show ip eigrp interfaces detail <name>` 命令却可以用于对路由器接口是否在发出多播数据包来发现和维护邻居关系,进行检查**。下面演示了在一台开启了动态邻居发现的路由器上该命令的输出:
```console
R2#show ip eigrp interfaces detail FastEthernet0/0
IP-EIGRP interfaces for process 150
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Fa0/0 1 0/0 1 0/1 50 0
Hello interval is 5 sec
Next xmit serial <none>
Un/reliable mcasts: 0/2 Un/reliable ucasts: 2/2
Mcast exceptions: 0 CR packets: 0 ACKs suppressed: 0
Retransmissions sent: 1 Out-of-sequence rcvd: 0
Authentication mode is not set
Use multicast
```
> **注意**`show ip eigrp neighbours`命令将在后面讲到。在查看`show ip eigrp interfaces detail <name>`命令的输出时要注意因为EIGRP同时用到多播及单播数据包both Multicast and Unicast packets所以该命令的计数器将包含两种类型数据包的数值如上面输出所示。
### 静态的邻居发现
**Static Neighbour Discovery**
与动态EIGRP邻居发现过程不同静态EIGRP邻居关系需要在路由器上手动配置邻居。在配置好静态EIGRP邻居后本地路由器就使用单播邻居地址往这些路由器发送数据包。
在EIGRP网络中静态邻居关系的使用是十分罕见的。这主要是因为手动邻居配置在大型网络中无法适应其规模。但重要的是弄明白**为何思科IOS软件中还是有此选项以及在什么情况下可以运用到此特性**。使用静态邻居配置的一个主要就是在那些没有广播或多播数据包原生支持的传输介质比如帧中继上部署EIGRP的情况下A prime example of when static neighbour configuration could be used would be in a situation where EIGRP is being deployed across media that does not natively support Broadcast or Multicast packets, such as Frame Relay
另一实例就是在譬如以太网这样的多路访问网络上仅有少数几台开启了EIGRP的路由器时为了防止发送不必要的EIGRP数据包的情形。在此情况下除开基本的EIGRP配置还必须在本地路由器上对**所有**静态EIGRP邻居配置`neighbour`命令。**如果一台路由器被配置为使用单播静态而其它路由器又被配置为使用多播动态那么这些开启了EIGRP的路由器是不会建立邻接关系的**EIGRP-enabled routers will not establish an adjacency if one router is configured to use Unicast(static) while another uses Multicast(dynamic))。
在思科IOS软件中静态EIGRP的邻居是通过使用路由器配置命令router configuration command`neighbour <address> <interface>`进行配置的。记住**这只是简单的对基础EIGRP配置的补充**this is simply in addition to the basic EIGRP configuration。下图36.5中给出的简单网络拓扑将用于静态EIGRP邻居的演示和验证。
![配置静态的EIGRP邻居](images/3605.png)
*图 36.5 -- 配置静态的EIGRP邻居*
参考图36.5中所给出的拓扑,路由器`R2`将作如下配置:
```console
R2(config)#router eigrp 150
R2(config-router)#network 192.168.1.0 0.0.0.255
R2(config-router)#neighbor 192.168.1.3 FastEthernet0/0
R2(config-router)#no auto-summary
R2(config-router)#exit
```
而应用于路由器`R3`上的配置则如下:
```console
R3(config)#router eigrp 150
R3(config-router)#network 192.168.1.0 0.0.0.255
R3(config-router)#neighbor 192.168.1.2 FastEthernet0/0
R3(config-router)#no auto-summary
R3(config-router)#exit
```
可使用`show ip eigrp interfaces detail <name>`命令,对路由器接口使用多播(动态),还是使用单播(静态)数据包来进行邻居发现与维护进行判断。下面的输出对此进行了演示:
```console
R2#show ip eigrp interfaces detail FastEthernet0/0
IP-EIGRP interfaces for process 150
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Fa0/0 1 0/0 2 0/1 50 0
Hello interval is 5 sec
Next xmit serial <none>
Un/reliable mcasts: 0/1 Un/reliable ucasts: 3/8
Mcast exceptions: 1 CR packets: 1 ACKs suppressed: 2
Retransmissions sent: 1 Out-of-sequence rcvd: 0
Authentication mode is not set
Use unicast
```
此外,可使用`show ip eigrp neighbours [detail]`命令来判断EIGRP邻居的类型。在本课程模块的后面将对此命令进行详细讲解。
### EIGRP的Hello及保持计时器
**EIGRP Hello and Hold Timers**
增强的IGRP对不同传输介质使用不同的Hello及保持计数器。Hello计时器被用于确定发送EIGRP Hello数据包的时间间隔。保持计数器则用于确定下路由器在认为其某个邻居宕机前要经历的时间the Hold timer is used to determine the time that will elapse before a router consider an EIGRP neighbour as down。默认保持时间是Hello间隔的3倍。
在广播网络、点对点串行网络、点对点子接口网络及高于T1线路速率的多点电路网络上增强的IGRP每5秒发送一次Hello数据包Enhanced IGRP sends Hello packets every 5 seconds on Broadcast, Point-to-Point Serial, Point-to-Point subinterfaces, and Multipoint circuits greater than T1 speed。而默认的保持时间就是15秒。在其它链路类型网络上包括速率低于T1线路的低带宽的WAN链路EIGRP每60秒发送Hello数据包。在这些链路上的邻居关系保持时间也是Hello间隔的3倍也就是180秒。
在那些相邻路由器上不必为了形成邻居关系而要求EIGRP计时器数值保持一致Enhanced IGRP timer value do not have to be the same on neighbouring routers in order for a neighbour relationship to be established。此外对于保持时间是Hello间隔的3倍这一点也没有强制性要求。这只是一种建议的做法a recommended guideline因此可在思科IOS软件中进行手动修改。可使用接口配置命令`ip hello-interval eigrp <ASN> <secs>`对EIGRP的Hello时间进行调整使用借口配置命令`ip hold-time eigrp <ASN> <secs>`对EIGRP的保持时间进行调整。
掌握EIGRP中Hello计时器及保持计时器的用法是重要的。保持时间是在EIGRP的Hello数据包中通告的同时Hello时间值告诉本地路由器往其邻居发送Hello数据包的频率。而保持时间则告诉其邻居路由器在等待多长时间后就可以宣布其已“死亡”the hold time, on the other hand, tells the neighbour router(s) of the local router how long to wait before declaring the local router "dead"。下图36.6演示了EIGRP的Hello数据包以及保持时间字段the Hold Time field:
![EIGRP Hello 数据包中的EIGRP保持时间](images/3606.png)
*图 36.6 -- EIGRP Hello 数据包中的EIGRP保持时间*
参考图36.6, 除开其它方面该EIGRP Hello数据包OPCode 5包含了所配置的保持时间数值。图36.6中所显示的值15, 是一个使用接口配置命令`ip hold-time eigrp <ASN> <secs>`所配置的非默认数值。重要的是记住在Hello数据包中是**不包含Hello时间间隔的**。但可使用`show ip eigrp interfaces detail <name>`命令查看到所配置的Hello时间。下面演示了此命令所打印出的信息
```console
R2#show ip eigrp interfaces detail FastEthernet0/0
IP-EIGRP interfaces for process 150
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Fa0/0 1 0/0 7 0/1 50 0
Hello interval is 5 sec
Next xmit serial <none>
Un/reliable mcasts: 0/1 Un/reliable ucasts: 2/5
Mcast exceptions: 1 CR packets: 1 ACKs suppressed: 0
Retransmissions sent: 1 Out-of-sequence rcvd: 0
Authentication mode is not set
Use multicast
```
调整默认EIGRP计时器数值的最常见原因就是要加速路由协议的收敛。比如在一条低速WAN链路上180秒的保持时间将是一个在EIGRP宣告某台邻居路由器宕机之前所要等待的相当长的时间。相反在某些情形中为了确保得到一个稳定的路由拓扑就可能有必要在某些高速链路上增加EIGRP计时器数值。在部署某种活动粘滞路由方案a solution for Stuck-In-Active routes这是常见的做法。本课程模块后面或详细讲解活动粘滞路由。
### EIGRP的邻居表
**EIGRP Neighbour Table**
运行EIGRP的路由器使用EIGRP邻居表来维护有关EIGRP邻居的状态信息。在学习到新近发现的邻居时该邻居的地址和接口就被记录下来。这对于动态发现的邻居与静态定义的邻居都是适用的。对每种协议相关模组Protocol-Dependent Module, PDM[Wikipedia上的PDM](https://en.wikipedia.org/wiki/Protocol-dependent_module)都有着一个唯一的EIGRP邻居表。
在某个EIGRP邻居发出了一个Hello数据包时其就通告出一个保持时间此保持时间就是某台路由器将一个邻居视为可达且运作中的时间长短。在某台路由器收到一个Hello数据包后该保持时间就开始减少直到计数到0。此时就会收到另一个Hello数据包同时保持时间又开始从头减少同时该过程继续重复。而如果在保持时间内未能收到Hello数据包那么保持时间就超时了到0。在保持时间超时后弥散更新算法就被告知拓扑的变化同时EIGRP宣告该邻居已宕机。路由器将打印并记录如下的一条类似消息
`%DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 10.1.1.2 (Serial0/0) is down: holding time expired`
EIGRP邻居表条目还包含了可靠传输协议the Reliable Transport Protocol, RTP所需要的信息。EIGRP使用可靠传输协议来确保更新、查询及应答数据包的可靠发送。此外还使用了顺序编号来匹配数据包与确认。EIGRP邻居表条目中记录了从该邻居收到的最后一个顺序编号以便检测出那些次序被打乱了的数据包In addtion, sequence numbers are also used to match acknowledgments with data packets. The last sequence number received from the neighbour is recorded in order to detect out-of-order packets。这样做确保了可靠的数据包送达。
> **注意:** 本课程模块后面详细讲到了RTP。
邻居表包含了每个邻居的一个在可能需要重传时用于对数据包进行排队的传输清单。此外在邻居数据结构中还有着一些往返计时器使用这些计时器来估算出最优重传间隔the neighbour table includes a transmission list that is used to queue packets for possible retransmission on a per-neighbour basis. Additionally, round-trip timers are kept in the neighbour data structure to estimate an optimal retransmission interval。所有这些信息都在`show ip eigrp neighbours`命令的输出中有打印出来。如下面所示:
```console
R2#show ip eigrp neighbors
IP-EIGRP neighbors for process 150
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 192.168.1.3 Fa0/0 14 00:43:08 2 200 0 12
```
对此命令所打印出的信息的掌握是相当重要的既是作为对一项核心EIGRP组件能力进行演示的基础同时也是对EIGRP故障进行排除的基础It is important to understand the information printed by this command, both as a basis for demonstrating competency on a core EIGRP component and for troubleshooting EIGRP issues。下表36.2对此命令输出中所包含的那些字段,进行了列出和说明:
*表 36.2 -- EIGRP邻居表的各个字段*
| 字段 | 说明 |
| :------ | :------ |
| H | 邻居清单编号以所学习到的先后顺序以“0”开始 |
| Address | 邻居的IP地址 |
| Interface | 经由其学习到邻居的那个接口the interface via which the neighbour is learned |
| Hold | 邻居的保持计数器在其到0时邻居就已宕机 |
| Uptime | 邻居关系已建立了多长时间 |
| SRTT | 平滑往返时间Smooth Round-Trip Time, 发送并接收到一个可靠EIGRP数据包所花费的时间 |
| RTO | 重传超时Retransmission Timeout, 在未收到一次确认时路由器重新传送EIGRP可靠数据包所要等待的时间 |
| Q Cnt | 等待发送中的EIGRP数据包Update, Query and Reply数量 |
| Sequence Number | 从该邻居所接收到的上一个EIGRP可靠数据包的顺序编号用以确保自该邻居接收到的数据包是有序的 |
尽管`show ip eigrp neighbours`命令打印出有关已知EIGRP邻居信息其在动态发现的邻居和手动配置的邻居上是没有区别的。比如在路由器`R2`上的该`show ip eigrp neighbours`命令的输出表明该路由器有着两个EIGRP邻居关系。在此配置下其中一个是静态配置的邻居而另一个则是动态发现的。可以看出从下面的输出是没法判断出哪个是哪个的
```console
R2#show ip eigrp neighbors
IP-EIGRP neighbors for process 150
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
1 150.2.2.2 Se0/0 13 00:00:48 153 918 0 4
0 192.168.1.3 Fa0/0 10 08:33:23 1 200 0 20
```
在路由器同时有着动态发现与静态配置的邻居关系环境中,可以使用`show ip eigrp neighbours detail`命令,来判断出哪个邻居是静态配置的,哪个是动态发现的,如下面所示:
```console
R2#show ip eigrp neighbors
IP-EIGRP neighbors for process 150
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
1 150.2.2.2 Se0/0 11 00:04:22 153 918 0 4
Version 12.3/1.2, Retrans: 0, Retries: 0, Prefixes: 1
0 192.168.1.3 Fa0/0 10 08:33:23 1 200 0 20
Static neighbour
Version 12.4/1.2, Retrans: 0, Retries: 0, Prefixes: 1
```
参考上面的输出,邻居`192.168.1.3`就是手动配置的邻居,而邻居`150.2.2.2`则是动态发现的邻居了。还可以通过使用`show ip eigrp neighbours static <interface>`,来查看到那些静态邻居,如下所示:
```console
R2#show ip eigrp neighbors static FastEthernet0/0
IP-EIGRP neighbors for process 150
Static Address Interface
192.168.1.3 FastEthernet0/0
```
### 可靠传输协议
**Reliable Transport Protocol**
增强的IGRP需要自己的传输协议来确保数据包的可靠送达。EIGRP使用可靠传输协议来确保更新Update、查询Query及应答Reply数据包的可靠发送。顺序编号的使用还确保了EIGRP数据包的有序接收。
当可靠EIGRP数据包发送到某个邻居时发送路由器期望从接收路由器收到一个表明该数据包已收到的确认。在使用可靠传输协议时EIGRP维护着一个未确认数据包的传输窗口a transport window of one unacknowledged packet, 这就意味着所发出的所有可靠数据包,都需要进行确认,之后才能发出下一个数据包。发送方路由器将对未收到确认的可靠数据包进行重传,直到其收到一个确认。
但重要的是需注意到,**未经确认的数据包只会重传16次**。如在16次重传后仍没有确认EIGRP将对该邻居关系进行重置。可靠传输协议使用到多播及单播数据包。在诸如以太网这样的广播多路访问网络EIGRP就会向网段上的每台路由器发送多播数据包而不是单个数据包单播On Broadcast Multi-Access networks such as Ethernet, EIGRP uses Multicast packets instead of sending an individual packet(Unicast) to each router on the segment。但在没有从多路访问网段上一台或更多的邻居收到响应时还是会用单播发送数据包。下面结合图36.7中的图表,对此进行了说明:
![EIGRP的可靠传输协议的运作](images/3607.png)
*图 36.7 -- EIGRP可靠传输协议的运作*
在图36.7中,路由器`R1`、`R2`与`R3`位于多路访问网段上的同一子网中。在给定的传输介质下EIGRP将使用多播在这些路由器直接发送可靠数据包。这里假定比如路由器`R1`发出了一个需要确认的数据包给路由器`R2`和`R3`。随后`R1`就等待来自`R2`和`R3`收到此数据包的确认。
假设路由器`R3`响应了,`R2`却无法对此数据包进行响应。在EIGRP维护了一个未确认数据包传输窗口的情况下就是说每个发出的单独可靠数据包在发送下一个可靠数据包之前都必须要邻居路由器进行显式的确认而由于路由器`R1`将无法在收到来自`R2`的确认前,发出数据包,这样就在该多路访问网段上出现了一个可能的问题。因此路由器`R3`就间接受到`R2`上故障的影响了。
为了避免这种坑,路由器`R1`将等待连接到该多路访问网段上的以太网接口的多播流计时器超时To avoid this potential pitfall, `R1` will wait for the Multicast Flow Timer(MFT) on the Ethernet interface connected to the Multi-access segment to expire。多播流计时器或简单的说就是流计时器the Flow Timer是发送方路由器等待自某个组成员的确认数据包的最长时间。在该计数器超时后路由器`R1`将以多播方式发出一个特殊的名为顺序TLV的EIGRP数据包when the timer expires, `R1` will Multicast a special EIGRP packet called a Sequence TLV。此数据包列出了路由器`R2`也就是例外的那台路由器the offender且表明其是一个顺序错乱的多播数据包this packet lists `R2`(the offender) and indicates an out-of-order Multicast packet。而因为路由器`R3`未被列入到该数据包所以其就进入到条件接收模式the Conditional Receive(CR) mode, 并继续侦听多播数据包。路由器`R1`此时就使用单播,将该数据包重传给`R2`。重传超时the Retransmission Timeout, RTO表示等待那个单播数据包的确认的时间。如在总共16次尝试后仍没有来自路由器`R2`的响应EIGRP将重置该邻居。
> **注意:** 当前的CCNA考试不要求对MFT及RTO有深入了解。
## 各种度量值、弥散更新算法及拓扑表
**Metrics, DUAL, and the Topology Table**
在部署EIGRP时对于路由被真正放入到IP路由表中之前所用到的EIGRP本身及为其所用到的方方面面的概念、方法及数据结构等的掌握是重要的。在本小节中将学到有关EIGRP的综合度量值及其计算方式。还将学习影响度量值计算及对计算出的度量值进行调整的不同方式when implementing EIGRP, it is important to understand **the various aspects used within and by the protocol before routes are actually placed into the IP routing table**. In this section, you will learn about **the EIGRP composite metric and how it is calculated**. You will also learn about **the different ways to influence metric calculation, as well as to adjust the calculated metric**)。
在那之后,将学习到**弥散更新算法**the Diffusing Update Algorithm, DUAL与**EIGRP的拓扑表**。此小节包括了一个有关如何在一台运行着EIGRP的路由器上将所有这些信息进行配合以最终产生出IP路由表的讨论。
### EIGRP综合度量值的计算
**EIGRP Composite Metric Calculation**
增强的EIGRP使用了一种综合度量值a composite metric, 该度量值包含了以不同的K值所表示的不同变量Enhanced IGRP uses a composite metric, which includes different variables referred to as the K values。这些K值是一些常量用于赋予路径的不同方面以不同的权重这些路径的不同方面都可能包含在该综合EIGRP度量值中。这些K值的默认值为 `K1=K3=1`, `K2=K4=K5=0`。也就是说K1与K3被默认被设置为1, 同时K2、K4和K5默认被设置为0。
假定在这些默认的K值下那么完整的EIGRP度量值就可以使用下面的数学公式算出来
`[K1*带宽 + (K2*带宽)/(256-负载) + K3*延迟] * [K5/(可靠性+K4)]`
但在仅有K1和K3有着默认的正值的情况下默认的EIGRP度量值是由下面的数学公式计算出来的
`[(10^7/路径上的最低带宽) + (所有延迟总和)] x 256`
这实际上就是说EIGRP使用了到目的网络的路径上的最小带宽以及总的累积延迟来计算理由度量值。不过思科IOS软件允许管理员将其它K值设置为非零值以将其它变量结合到该综合度量值中。通过使用路由器配置命令`metric weights [tos] k1 k2 k3 k4 k5`,就可完成此操作。
在使用`metric weights`命令时,`[tos]`表示服务类型Type of Service。尽管思科IOS软件显示可以使用任何0到8之间的数值但在撰写本手册时该字段`[tos]`当前却只能被设置为0。而这些K值就可以被设置为0到255之间的任何数值。通过执行`show ip protocols`命令就可查看默认的这些EIGRP K值。下面的输出对此进行了演示
```console
R2#show ip protocols
Routing Protocol is “eigrp 150”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 150
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is not in effect
Maximum path: 4
Routing for Networks:
192.168.1.0
Routing Information Sources:
Gateway Distance Last Update
192.168.1.3 90 00:00:15
Distance: internal 90 external 170
```
在对这些EIGRP的K值进行调整时重要的是记住在EIGRP域中的所有路由器上都要配置上同样的这些数值。如这些**K值不匹配那么EIGRP的邻居关系就不会建立**。
> **注意:** 不建议对这些默认的K值进行调整。对这些K值的调整只应在那些对网络中这类行为造成的后果有扎实了解老练的高级工程师的指导下或在思科公司技术支持中心的建议下完成。
### 使用接口带宽来影响EIGRP的度量值
**Using Interface Bandwidth to Influence EIGRP Metric Calculation**
可通过使用`bandwidth`命令对指定到那些单个接口的默认带宽进行调整从而直接对EIGRP度量值计算施加影响。**通过此命令指定的带宽是以千位Kilobits计的**。在EIGRP的度量值计算中带宽也是以千位计的。下图36.8演示了一个由两台路由器通过两条带宽为1544Kbps, 的串行T1链路连接所组成的网络。
![EIGRP度量值的带宽修改](images/3608.png)
*图 36.8 -- EIGRP度量值的带宽修改*
参考图36.8中的图示,因为路由器`R1`与`R2`之间两条链路的带宽(及延迟)是相等的,所以从路由器`R2`到子网`172.16.100.0/24`将同时继承到这两条路径的相同EIGRP度量值because of the equal bandwidth (and delay) values of the links between `R1` and `R2`, the same EIGRP metric will be derived for both paths from `R2` to the `172.16.100.0/24` subnet。EIGRP将在这两条链路之间进行流量负载均衡如下面路由器`R2`上的输出所示:
```console
R2#show ip route 172.16.100.0 255.255.255.0
Routing entry for 172.16.100.0/24
Known via “eigrp 150”, distance 90, metric 2172416, type internal
Redistributing via eigrp 150
Last update from 150.2.2.1 on Serial0/1, 00:48:09 ago
Routing Descriptor Blocks:
150.2.2.1, from 150.2.2.1, 00:48:09 ago, via Serial0/1
Route metric is 2172416, traffic share count is 1
Total delay is 20100 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
* 150.1.1.1, from 150.1.1.1, 00:48:09 ago, via Serial0/0
Route metric is 2172416, traffic share count is 1
Total delay is 20100 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops
```
对两条链路中的任何一条的带宽进行调整都会直接影响到EIGRP对到目的网络路径的度量值计算。**这样的操作,可用于更为大型网络中路径的控制(也就是基于管理员定义的数值与配置,对流量所要采行的路径进行控制)**Such actions can be used for path control within larger networks(i.e., controlling the path that traffic takes based on administrator-defined values and configurations)。比如这里要令到EIGRP优先使用`Serial0/0`作为前往目的网络的主要路径,而将`Serial0/1`作为到目的网络的备份路径,就要采取两种操作之一。
第一种操作,就是可以增加`Serial0/0`上的带宽值,造成该路径的一个更好(更低)的度量值。那么第二种方法,就是可以降低`Serial0/1`上的带宽值,造成该路径的一个更差(更高)的度量值。两种选项都是可接受的,同时都将达成所需的结果。下面的输出演示了如何将`Serial0/0`上的默认带宽进行降低,从而有效地确保`Serial0/0`作为路由器`R2`及`172.16.100.0/24`网络之间的主要路径。
```console
R2(config)#interface Serial0/1
R2(config-if)#bandwidth 1024
R2(config-if)#exit
```
> **注意:** 如同在第1天指出的该配置并不意味着接口`Serial0/1`上仅容许1024Kbps速率的流量通过该接口As stated in Day 1, this configuration does not mean that `Serial0/1` is now capable of only 1024Kbps of throughput through this interface
该配置的结果就是接口`Serial0/0`成为路由器`R2`到达目的网络`172.16.100.0/24`网络的主要路径。这在下面的输出中有所演示:
```console
R2#show ip route 172.16.100.0 255.255.255.0
Routing entry for 172.16.100.0/24
Known via “eigrp 150”, distance 90, metric 2172416, type internal
Redistributing via eigrp 150
Last update from 150.1.1.1 on Serial0/0, 00:01:55 ago
Routing Descriptor Blocks:
* 150.1.1.1, from 150.1.1.1, 00:01:55 ago, via Serial0/0
Route metric is 2172416, traffic share count is 1
Total delay is 20100 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
```
> **注意:** 这里星号the asterisk, `*`)指向的接口,就是下一数据包要发送出去的接口。而在路由表中有着多个开销相等的路由时,星号的位置就会在这些开销相等的路径之间轮转。
在将EIGRP作为路由协议时尽管经由`Serial0/1`接口的路径未被安装到**路由表**中, 重要的是记住该路径并未被完全忽略掉Although the path via the `Serial0/1` interface is not installed into the routing table, when using EIGRP as the routing protocol, it is important to remember that this path is not completely ignored。而是该路径被存储在**EIGRP的拓扑表**中EIGRP的拓扑表包含了到那些远端目网络的主要及替代备份路径。本课程模块后面将对EIGRP的拓扑表予以讲解。
> **注意:** 默认在开启了EIGRP时其可能会用到高达接口带宽的50%来发送EIGRP本身的数据包EIGRP是一种非常话痨的协议所以其在可能的带宽使用上进行了限制EIGRP is a very chatty protocol, so it limits itself in possible bandwidth usage。EIGRP是基于接口配置命令`bandwidth`,来判断带宽数量的。因此在对接口带宽数值进行调整时,就要记住这点。而该默认设置,可使用接口配置命令**`ip bandwidth-percent eigrp [ASN] [percentage]`**,进行修改。
总的来说,在应用带宽命令`bandwidth`对EIGRP的度量值计算施加影响时重要的是记住EIGRP会使用到目的网络路径上的最小带宽以及延迟的累计值来计算路由度量值EIGRP uses the minimum bandwidth on the path to a destination network, along with the cumulative delay, to compute routing metrics。同时还要对网络拓扑有牢固掌握以对在何处使用`bandwidth`命令从而实现对EIGRP度量值计算的影响。**但在真实世界中对EIGRP度量值施加影响的首选方法不是修改带宽而是修改延迟**。
### 运用接口的延迟来对EIGRP的度量值计算进行影响
**Using Interface Delay to Influence EIGRP Metric Calculation**
接口延迟数值是用微秒来表示的。但在EIGRP度量值计算中用到的延迟数值是以10微秒计的in tens of microseconds。因此为了计算出EIGRP的度量值接口上的延迟数值就必须除以10。下面的表36.3对思科IOS软件中使用到的默认接口带宽及延迟数值进行了演示
*表 36.3 -- 默认的接口带宽与延迟数值*
| 接口类型 | 带宽Kilobits | 延迟Microseconds, us |
| :-------- | --------: | --------: |
| 以太网Ethernet | 10000 | 1000 |
| 快速以太网FastEthernet | 100000 | 100 |
| 千兆以太网GigabitEthernet | 1000000 | 10 |
| 万兆以太网Ten-GigabitEthernet | 10000000 | 10 |
| 串行线路T1 | 1544 | 20000 |
| 串行线路E1 | 2048 | 20000 |
| 串行线路T3 | 44736 | 200 |
| 串行线路E3 | 34010 | 200 |
在对接口的带宽及延迟数值进行计算时,重要的是记住**对接口带宽的调整并不会自动地调整到接口的延迟,相反也是这样。这两个数值是相互独立的**。比如,下面的输出展示了一个快速以太网接口的默认带宽及延迟数值:
```console
R2#show interfaces FastEthernet0/0
FastEthernet0/0 is up, line protocol is up
Hardware is AmdFE, address is 0013.1986.0a20 (bia 0013.1986.0a20)
Internet address is 192.168.1.2/24
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
...
[Truncated Output]
```
为对此概念进行强化,下面使用接口配置命令`bandwidth`将该快速以太网接口的带宽调整为1544Kbps
```console
R2(config)#interface FastEthernet0/0
R2(config-if)#bandwidth 1544
R2(config-if)#exit
```
此时显示在`show interfaces`命令的输出中的带宽数值,反应了该已应用下去的配置,但默认的接口延迟数值却仍然保持原来的大小,如下面的输出所示:
```console
R2#show interfaces FastEthernet0/0
FastEthernet0/0 is up, line protocol is up
Hardware is AmdFE, address is 0013.1986.0a20 (bia 0013.1986.0a20)
Internet address is 192.168.1.2/24
MTU 1500 bytes, BW 1544 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
```
而EIGRP所使用的累积延迟则是在源网络与目的网络之间所有接口延迟的和。对路径中任何一个延迟数值的修改都会影响到EIGRP度量值的计算。接口延迟数值是通过使用接口配置命令`delay`进行调整的。该数值在用于EIGRP度量值计算时会除以10。下图36.9演示了一个由两台通过两条有着1544Kbps的带宽及默认20000微秒的延迟串行T1链路连接的路由器所组成的网络。此外网络`172.16.100.0/24`是直接连接到一个快速以太网接口的该以太网接口有着默认100000Kbps的带宽以及默认100微秒的延迟数值
![EIGRP度量值中延迟的修改](images/3609.png)
*图 36.9 -- EIGRP度量值中延迟的修改*
那么从路由器`R2`到网络`172.16.100.0/24`的EIGRP度量值的计算如下
度量值 = [(10^7/路径上的最小带宽) + (延迟综合)] x 256
度量值 = [(10000000/1544) + (2000+10)] x 256
> **注意:** 记住在EIGRP度量值计算中要将接口延迟数值除以10。
> **注意:** 这里计算出的数值应总是要向下取到最接近的整数。
度量值 = [(10000000/1544) + (2000+10)] x 256
度量值 = [6476 + 2010] x 256
度量值 = 8486 x 256
度量值 = 2172416
可使用`show ip route`命令对此计算进行验证,如下所示:
```console
R2#show ip route 172.16.100.0 255.255.255.0
Routing entry for 172.16.100.0/24
Known via “eigrp 150”, distance 90, metric 2172416, type internal
Redistributing via eigrp 150
Last update from 150.2.2.1 on Serial0/1, 00:03:28 ago
Routing Descriptor Blocks:
150.2.2.1, from 150.2.2.1, 00:03:28 ago, via Serial0/1
Route metric is 2172416, traffic share count is 1
Total delay is 20100 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
* 150.1.1.1, from 150.1.1.1, 00:03:28 ago, via Serial0/0
Route metric is 2172416, traffic share count is 1
Total delay is 20100 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
```
与使用`bandwidth`命令一样为了对EIGRP的度量值计算施加影响我们既可以使用`delay`命令对接口延迟数值进行提升,也可以对其进行降低。比如,为了将路由器`R2`配置为使用链路`Serial0/0`到达`172.16.100.0/24`网络,而将`Serial0/1`仅用作一条备份链路,那么就可以如下将`Serial0/0`上的延迟数值进行降低:
```console
R2(config)#int s0/0
R2(config-if)#delay 100
R2(config-if)#exit
```
此配置就对经由`Serial0/0`的路径的EIGRP度量值进行了调整如下所示
```console
R2#show ip route 172.16.100.0 255.255.255.0
Routing entry for 172.16.100.0/24
Known via “eigrp 150”, distance 90, metric 1686016, type internal
Redistributing via eigrp 150
Last update from 150.1.1.1 on Serial0/0, 00:01:09 ago
Routing Descriptor Blocks:
* 150.1.1.1, from 150.1.1.1, 00:01:09 ago, via Serial0/0
Route metric is 1686016, traffic share count is 1
Total delay is 1100 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
```
而经由`Serial0/1`的路径则被保留在拓扑表中作为到该网络的一条替代路径an alternate path
### 关于弥散更新算法
**The Diffusing Update Algorithm(DUAL)**
本小节涉及以下术语:
- 可行距离the Feasible Distance
- 后继路由器the Successor
- 报告的距离the Reported Distance
- 通告的距离, the Advertised Distance, 与报告的距离一样
- 可行后继the Feasible Successor
- 可行条件the Feasible Condition
弥散更新算法是EIGRP路由协议的关键所在。弥散更新算法会对从邻居路由器处接收到的所有路由进行观察与比较然后选出有着到目的网络最低度量值最优--该度量值就是可行距离the Feasible Distance, FD--的无环回路径从而得到后续路由the Successor route。可行距离既包含了由相连的路由所通告的某网络的度量值还要加上到那台特定邻居路由器的开销。
由邻居路由器所通告的度量值就被叫做到目的网路报告的距离the Reported Distance, RD, 又被叫做到目的网络通告的距离the Advertised Distance, AD, 所以 RD == AD。因此可行距离就包含了报告的距离加上到达那台特定邻居路由器的开销。那么该后续路由的下一跳路由器就被叫做是后续路由器the Successor。后续路由器就被放入到IP路由表the IP routing table及EIGRP拓扑表the EIGRP topology table中, 并指向到后继路由器。
到相同目的网络的其它那些有着比起后继路由器路径的可行距离高一些的报告距离却仍然是无环回的那些路由就被叫做是可行后继路由器路由了Any other routes to the same destination network that have a lower RD than the FD of the Successor path are guaranteed to be loop-free and are referred to as Feasible Successor(FS) routes, 这里疑似原作者笔误“lower”应该是"higher"。这些路由不会被放入到IP路由表中但它们仍然会与其它后继路由器路由一起被放入到EIGRP的拓扑表。
为了某条路由成为可行后继路由则其必须满足可行性条件the Feasible Condition, FC, 该可行性条件仅会在到目的网络的报告距离少于可行距离时才会发生。在报告距离高于可行距离时该路由不会被选作可行后继FS。EIGRP这么做是为了防止可能出现的环回。下图36.10中所演示的网络拓扑,将会用于对本小节中出现的术语进行解释。
![掌握弥散更新算法](images/3610.png)
*图 36.10 -- 掌握弥散更新算法*
参考图36.10, 下表36.4给出了在路由器`R1`上观察到的到`192.168.100.0/24`网络的可行距离及报告距离数值:
*表 36.4 -- `R1`的那些路径及距离, `R1` Paths and Distances*
| 网络路径 | `R1` 的邻居 | 邻居度量值(报告距离) | `R1`的可行距离 |
| :------- | :------- | -------: | -------: |
| `R1`-`R2`-`R5` | `R2` | 30 | 35 |
| `R1`-`R3`-`R5` | `R3` | 10 | 30 |
| `R1`-`R4`-`R5` | `R4` | 15 | 25 |
基于表36.4中的信息,路由器`R1`将选择经由`R4`的路径作为后继路由这是根据该路由的可行距离得出的。此路由将被放入到IP路由表以及EIGRP拓扑表中。路由器`R1`随后将对那些到`192.168.100.0/24`网络的替代路径进行查看。这里邻居路由器`R3`到`192.168.100.0/24`网络的度量值又被叫做是报告的距离或通告距离就是10。该距离小于当前的可行距离所以该路由满足到可行条件FC那么就被放入到EIGRP的拓扑表中。而邻居路由器`R2`到`192.168.100.0/24`的度量值为30。该值高于了当前的可行距离25。此路由则不能满足可行条件就不被看作是一个可行后继FS。但该路由仍然会被放入到EIGRP的拓扑表中。这将在后面的EIGRP拓扑表小节进行演示。
当某个邻居路由器改变了度量值或拓扑发生了改变以及后继路由被移除或改变时弥散更新算法会检查那些可行后继路由器的路由在发现了一台可行后继路由器时弥散更新算法就使用该可行后继路由器以避免不必要的重新计算路由。执行一次本地运算节省了CPU处理能力因为在当前后继或主路由失效时可行后继路由本身就已选出且已经存在了。此过程就叫做**本地运算**When a neighbor changes a metric, or when a topology change occurs, and the Successor route is removed or changes, DUAL checks for FSs for the route and if one is found, then DUAL uses it to avoid re-computing the route unnecessarily. This is referred to as local computation. Performing **a local computation** saves CPU power because the FS has been chosen and already exists before the Successor or primary route fails
而当目的网络的可行后继路由器不存在时,本地路由器将向邻居路由器发出一次查询,对邻居路由器是否有着关于目的网络的信息。如邻居路由器有该信息,同时另一路由器确实有着到目的网络的路由,此时该路由器将执行一次**弥散运算**以确定出一台新的后继路由器If the information is available and another neighbour does have a route to the destination network, then the router performs **a diffusing computation** to determine a new Successor
## EIGRP的拓扑表
**The EIGRP Topology Table**
EIGRP的拓扑表是由EIGRP的各种**协议相关模块**, 在**弥散更新算法的有限状态机**之上运算得到的The EIGRP topology table is populated by **EIGRP PDMs** acted upon by **the DUAL Finite State Machine**。由已形成邻居关系的EIGRP路由器neighbouring EIGRP routers通告的所有目的网络与子网都被存储在EIGRP拓扑表中。该表包含了后继路由器路由、可行后继路由器路由甚至那些并不满足可行条件的路由This includes Successor routes, FS routes, and even routes that have not met the FC
正是拓扑表才令到所有EIGRP路由器对整个网络有着一致的视图。其还实现了EIGRP网络中的快速收敛。在拓扑表中的每个条目都包含了某个目的网络及那些通告该目的网络的**那些邻居**。可行距离及通告距离都被存储在拓扑表中。EIGRP的拓扑表包含了构建出一个到所有可达网络的距离与矢量集合的所需信息The topology table allows all EIGRP routers to have a consistent view of the entire network. It also allows for rapid convergence in EIGRP networks. Each individual entry in the topology table contains the destination network and the neighbour(s) that have advertised the destination network. Both the FD and the RD are stored in the topology table. The EIGRP topology table contains the information needed to build a set of distances and vectors to each reachable network这些信息包括以下内容
- 到目的网络的最低带宽, the lowest bandwidth on the path to the destination network
- 到目的网络的总/累积延迟, the total or cumulative delay to the destination network
- 到目的网络路径的可靠性, the reliability of the path to the destination network
- 到目的网络路径的负载the loading of the path to the destination network
- 到目的网络的最大传输单元的最小值, The minimum Maximum Transmission Unit(MTU) to the destination network
- 到目的网络的可行距离, the Feasible Distance to the destination network
- 由邻居路由器所报告的到目的网络的报告距离, the Reported Distance by the neighbour router to the destination network
- 目的网络的路由源仅针对那些外部路由The route source(only external routes) of the destination network
> **注意:** 尽管在拓扑表中包含了最大传输单元MTU但EIGRP并不会在实际的度量值计算中使用到该数值。而是该MTU仅简单地作为判断到目的网络数据包大小最小值而被追踪。接口的最大传输单元指定了经某条链路在无需将数据报或数据包拆分到更小片的情况下所能传输的数据报最大大小The interface MTU specifies the largest size of datagram that can be transferred across a certain link without the need of fragmentation, or breaking the datagram or packet into smaller pieces
使用`show ip eigrp topology`命令就可查看到EIGRP拓扑表的内容。该命令下可用的选项如下所示
```console
R2#show ip eigrp topology ?
<1-65535> AS Number
A.B.C.D IP prefix <network>/<length>, e.g., 192.168.0.0/16
A.B.C.D Network to display information about
active Show only active entries
all-links Show all links in topology table
detail-links Show all links in topology table
pending Show only entries pending transmission
summary Show a summary of the topology table
zero-successors Show only zero successor entries
| Output modifiers
<cr>
```
不带选项的`show ip eigrp topology`命令将仅打印出那些拓扑表中路由的、且是该路由器上所有开启的EIGRP实例的后继路由器及可行后继的信息The `show ip eigrp topology` command with no options prints only the Successor and Feasible Successor information for routes in the topology table and for all of the EIGRP instances enabled on the router。下面演示了该命令的打印输出
```console
R2#show ip eigrp topology
IP-EIGRP Topology Table for AS(150)/ID(2.2.2.2)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 150.2.2.0/24, 1 successors, FD is 20512000
via Connected, Serial0/1
via 150.1.1.1 (2195456/2169856), Serial0/0
P 150.1.1.0/24, 1 successors, FD is 1683456
via Connected, Serial0/0
P 172.16.100.0/24, 1 successors, FD is 1686016
via 150.1.1.1 (1686016/28160), Serial0/0
```
而`show ip eigrp topology [network]/[prefix]`及`show ip eigrp topology [network] [mask]`两个命令则将打印出其各自所指定路由的后继路由、可行后继路由以及未能满足可行条件的那些其它路由The `show ip eigrp topology [network]/[prefix]` and `show ip eigrp topology [network] [mask]` commands print Successor routes, FS routes, and routes that have not met the FC for the route specified in either command。下面的输出演示了`show ip eigrp topology [network]/[prefix]`命令的用法:
```console
R2#show ip eigrp topology 172.16.100.0/24
IP-EIGRP (AS 150): Topology entry for 172.16.100.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 1686016
Routing Descriptor Blocks:
150.1.1.1 (Serial0/0), from 150.1.1.1, Send flag is 0x0
Composite metric is (1686016/28160), Route is Internal
Vector metric:
Minimum bandwidth is 1544 Kbit
Total delay is 1100 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
150.2.2.1 (Serial0/1), from 150.2.2.1, Send flag is 0x0
Composite metric is (2167998207/2147511807), Route is Internal
Vector metric:
Minimum bandwidth is 128 Kbit
Total delay is 83906179 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
```
在上面的输出中,可以看出经由`Serial0/1`的路径并没有满足可行条件FC因为其报告的距离RD超过了可行距离FD。这就是该路径没有在`show ip eigrp topology`命令的输出中打印出来的原因。而为了判断出那些后继路由、可行后继路由,以及未能满足可行条件的那些路由,就可以使用`show ip eigrp topology all-links`命令而不是对单个前缀进行查看从而查看到在EIGRP拓扑表中所有前缀的所有可能的路由。下面对此命令的输出进行了演示
```console
R2#show ip eigrp topology all-links
IP-EIGRP Topology Table for AS(150)/ID(2.2.2.2)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 150.2.2.0/24, 1 successors, FD is 20512000, serno 42
via Connected, Serial0/1
via 150.1.1.1 (2195456/2169856), Serial0/0
P 150.1.1.0/24, 1 successors, FD is 1683456, serno 32
via Connected, Serial0/0
via 150.2.2.1 (21024000/2169856), Serial0/1
P 172.16.100.0/24, 1 successors, FD is 1686016, serno 47
via 150.1.1.1 (1686016/28160), Serial0/0
via 150.2.2.1 (2167998207/2147511807), Serial0/1
```
在该EIGRP拓扑表中的路由条目可能被标记为被动的Passive, P或主动的Active, A状态。处于被动状态的某条路由表明EIGRP已经完成了该路由度量值的主动计算同时可以使用该后继路由将流量转发到目的网络。此状态是拓扑表中所有路由的首选状态。
当后继路由丢失且路由器为确定出可行后继而发出了一个查询数据包时那些EIGRP的路由就处于主动状态了。通常情况下是存在着可行后继的且EIGRP会将那个可行后继提升为后继路由。那么在此情况下就无需涉及到网络中其它路由器该路由器就可以完成收敛。此过程就叫做一次本地运算Enhanced IGRP routes are in Active state when the Successor route has been lost and the router sends out a Query packet to determine an FS. Usually, an FS is present and EIGRP promotes that to the Successor route. This way, the router converges without involving other routers in the network. This process is referred to as **a local computation**)。
不过如后继路由已丢失或被移除而又没有可行后继时那么路由器就将开始弥散运算diffused computation。在弥散运算中EIGRP将往所有邻居路由器、从除开连接到后继路由的接口外的所有接口发出一次查询。当某台EIGRP邻居路由器收到某条路由的查询时如那个邻居的EIGRP拓扑表中没有包含该被查询路由的条目那么这个邻居就立即对该查询应答一条不可达报文指出经过此邻居处并无这条路由的路径。
而如果邻居路由器上的EIGRP拓扑表已将发出查询的路由器列为所查询路由的后继路由器同时存在一条可行后继路由那么该可行后继路由器路由就被安装起来同时该邻居路由器对该邻居查询做出其有着一条到已丢失目的网络路由的应答If the EIGRP topology table on the neighbour lists the router sending the Query as the Successor for that route, and an FS exists, then the FS is installed and the router replies to the neighbour Query that it has a route to the lost destination network
但如果该EIGRP拓扑表虽然将发出查询的路由器列为了该路由的后继路由器却没有可行后继时收到查询的路由器就会查询其所有的EIGRP邻居除开那些作为其先前后继路由器而发出的同样接口。在尚未收到对此路由的所有查询的一条应答时该路由器不会对先前的查询进行响应However, if the EIGRP topology table lists the router sending the Query as the Successor for this route and there is no FS, then the router queries all of its EIGRP neighbors, except those that were sent out of the same interface as its former Successor. The router will not reply to the Query until it has received a Reply to all Queries that it originated for this route
最后如某个非此目的网络后继的邻居收到了此次查询随后该路由器以其自己的后继信息予以了应答。而假如这些邻居路由器仍然没有该已丢失的路由信息那么这些邻居路由器就会向它们自己的邻居路由器发出查询直到抵达查询边界。所谓查询边界既可以是网络的末端、分发清单的边界或者汇总的边界Finally, if the Query was received from a neighbour that is not the Successor for this destination, then the router replies with its own Successor information. If the neighbouring routers do not have the lost route information, then Queries are sent from those neighbouring routers to their neighbouring routers until the Query boundary is reached. The Query boundary is either the end of the network, the distribute list boundary, or the summarization boundary
查询一旦发出那么发出查询的EIGRP路由器就必须在计算后继路由前等待完成所有应答的接收。如有任何邻居在三分钟之内没有应答那么该路由就被称作处于活动粘滞状态If any neighbour has not replied within three minutes, the route is said to be Stuck-In-Active(SIA))。而当某条路由成为活动粘滞路由时,该(这些)未对查询进行响应的路由器的邻居关系,就将被重置。在此情况下,可以观察到路由器记录下了如下类似的一条消息:
```console
%DUAL-5-NBRCHANGE: IP-EIGRP 150:
Neighbor 150.1.1.1(Serial0/0) is down: stuck in active
%DUAL-3-SIA:
Route 172.16.100.0/24 stuck-in-active state in IP-EIGRP 150.
Cleaning up
```
造成这些EIGRP邻居路由器未能对查询进行响应的原因有几种包括下面这些
- 该邻居路由器的CPU过载了因此而无法及时响应
- 该邻居路由器本身就没有关于那条丢失路由的信息
- 电路质量问题,造成数据包丢失
- 某些低带宽链路拥塞,从而造成数据包的延迟
而为了防止因为延迟响应造成的来自其它EIGRP邻居的活动粘滞方面的故障可使用路由器配置模式中的`timers active-time`命令,将本地路由器配置为等待多于默认的三分钟,以接收到返回给其查询数据包的响应。
> **注意:** 重要的是应注意在对网络中某台路由器上的该默认参数进行修改时就必须对EIGRP路由域中的所有路由器上的该参数进行修改It is important to note that if you change this default parameter on one EIGRP router in your network, you must change it on all the other routers within your **EIGRP routing domain**)。
## 相等开销及不相等开销下的负载均衡
**Equal Cost and Unequal Cost Load Sharing**
思科IOS软件对所有路由协议支持默认下至多4条路径的相等开销负载均衡Cisco IOS software supports equal cost load sharing for a default of up to four paths for all routing protocols。下面的`show ip protocols`命令的输出,对此进行了演示:
```console
R2#show ip protocols
Routing Protocol is “eigrp 150”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 150
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is not in effect
Maximum path: 4
Routing for Networks:
150.1.1.2/32
150.2.2.2/32
Routing Information Sources:
Gateway Distance Last Update
Gateway Distance Last Update
150.2.2.1 90 00:00:52
150.1.1.1 90 00:00:52
Distance: internal 90 external 170
```
而又可以使用路由器配置命令`maximum-paths <1-6>`对默认的最多4条路径修改为最多6条相等开销的路径。在进行相等开销的负载均衡时路由器将负载在所有路径直接进行均匀地分配。有着一个流量分享计数对每条路径上传输的数据包进行识别The traffic share count identifies the number of outgoing packets on each path。在进行相等开销的负载均衡时单个的完整数据包是在某条单独路径上发出的如下面的输出所示
```console
R2#show ip route 172.16.100.0 255.255.255.0
Routing entry for 172.16.100.0/24
Known via “eigrp 150”, distance 90, metric 2172416, type internal
Redistributing via eigrp 150
Last update from 150.2.2.1 on Serial0/1, 00:04:00 ago
Routing Descriptor Blocks:
150.2.2.1, from 150.2.2.1, 00:04:00 ago, via Serial0/1
Route metric is 2172416, traffic share count is 1
Total delay is 20100 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
* 150.1.1.1, from 150.1.1.1, 00:04:00 ago, via Serial0/0
Route metric is 2172416, traffic share count is 1
Total delay is 20100 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
```
除了相等开销下的负载均衡能力EIGRP还能完成不相等开销下的负载均衡。这种特别的能力令到EIGRP能够使用那些不相等开销的路径基于不同的流量分享权重数值从而将传输中的数据包发送到目的网络This unique ability allows EIGRP to use unequal cost paths to send outgoing packets to the destination network based on weighted traffic share values。通过使用路由器配置命令`variance <multiplier>`,就可以开启不相等开销下的负载均衡特性。
关键字`<multiplier>`是一个介于1到128之间的整数。默认的倍数multiplier1就是说不进行不相等开销下的负载均衡。此默认设置在下面的`show ip protocols`命令的输出中进行了演示:
```console
R2#show ip protocols
Routing Protocol is “eigrp 150”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 150
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is not in effect
Maximum path: 4
Routing for Networks:
150.1.1.2/32
150.2.2.2/32
Routing Information Sources:
Gateway Distance Last Update
150.2.2.1 90 00:00:52
150.1.1.1 90 00:00:52
Distance: internal 90 external 170
```
该倍数是一个可变的整数, 告诉路由器在那些有着低于最小度量值乘以该倍数的那些路由上进行负载均衡The multiplier is a variable integer that tells the router to load share across routes that have a metric that is less than the minimum metric multiplied by the multiplier。比如在指定了5的变化时specifying a variance of 5就指示路由器在那些度量值低于最小度量值5倍的路由上进行负载均衡。在使用了`variance`命令同时指定了除开1之外的倍数时该路由器就将在这些满足条件的路由上按照各条路由的度量值成比例的分配流量。也就是说对于那些有着较低度量值的路由比起那些有着较高度量值的路由要经由其发送更多的流量。
下图36.11演示了一个基本的运行EIGRP的网络。其中的路由器`R1`和`R2`,是通过背靠背的串行链路连接起来的(`R1` and `R2` are connected via back-to-back Serial links。两台路由器间的链路`150.1.1.0/24`有着1024Kbps的带宽。两台路由器之间的`150.2.2.0/24`链路有着768Kpbs的带宽。路由器`R1`是通过EIGRP将`172.16.100.0/24`前缀,通告给`R2`的。
![掌握EIGRP的非等价负载均衡](images/3611.png)
*图 36.11 -- 掌握EIGRP的非等价负载均衡Understanding EIGRP Variance*
基于图36.11中所演示的拓扑,下面的输出演示在路由器`R2`上`172.16.100.0/24`前缀的路由表:
```console
R2#show ip route 172.16.100.0 255.255.255.0
Routing entry for 172.16.100.0/24
Known via “eigrp 150”, distance 90, metric 3014400, type internal
Redistributing via eigrp 150
Last update from 150.1.1.1 on Serial0/0, 00:00:11 ago
Routing Descriptor Blocks:
* 150.1.1.1, from 150.1.1.1, 00:00:11 ago, via Serial0/0
Route metric is 3014400, traffic share count is 1
Total delay is 20100 microseconds, minimum bandwidth is 1024 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
```
下面的EIGRP拓扑表同时显示了后继与可行后继路由
```console
R2#show ip eigrp topology 172.16.100.0 255.255.255.0
IP-EIGRP (AS 150): Topology entry for 172.16.100.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 3014400
Routing Descriptor Blocks:
150.1.1.1 (Serial0/0), from 150.1.1.1, Send flag is 0x0
Composite metric is (3014400/28160), Route is Internal
Vector metric:
Minimum bandwidth is 1024 Kbit
Total delay is 20100 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
150.2.2.1 (Serial0/1), from 150.2.2.1, Send flag is 0x0
Composite metric is (3847680/28160), Route is Internal
Vector metric:
Minimum bandwidth is 768 Kbit
Total delay is 20100 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
```
这里为了确定要配置到路由器上的非等价负载均衡数值the variance value to configure on the router, 就可使用下面的公式:
非等价负载均衡数值Variance = 考虑要用到路径的度量值中最高的/最优路径的度量值(最小的度量值)
使用此公式,就可以像下面这样,计算出在路由器`R2`上,所要配置的非等价负载均衡数值:
非等价负载均衡数值Variance = 考虑要用到路径的度量值中最高的/最优路径的度量值(最小的度量值)
非等价负载均衡数值Variance = 3847680/3014400
非等价负载均衡数值Variance = 1.28
随后**必须对此数值进行向上取整**这里就是2了。那么就可以在路由器配置模式中通过应用将下面的配置将路由器`R2`配置为进行非等价的负载均衡了:
```console
R2(config)#router eigrp 150
R2(config-router)#variance 2
R2(config-router)#exit
```
而根据此配置,此时`172.16.100.0/24`前缀的路由表条目就变成如下所示了:
```console
R2#show ip route 172.16.100.0 255.255.255.0
Routing entry for 172.16.100.0/24
Known via “eigrp 150”, distance 90, metric 3014400, type internal
Redistributing via eigrp 150
Last update from 150.2.2.1 on Serial0/1, 00:00:36 ago
Routing Descriptor Blocks:
150.2.2.1, from 150.2.2.1, 00:00:36 ago, via Serial0/1
Route metric is 3847680, traffic share count is 47
Total delay is 20100 microseconds, minimum bandwidth is 768 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
* 150.1.1.1, from 150.1.1.1, 00:00:36 ago, via Serial0/0
Route metric is 3014400, traffic share count is 60
Total delay is 20100 microseconds, minimum bandwidth is 1024 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
```
其中的流量分配计数The traffic share count表明每从`Serial0/0`转发60个数据包路由器就将从`Serial0/1`转发47个数据包。数据包的转发是依两条路径的路由度量值的比例完成的。这是在应用了`variance`命令后的默认行为。而**通过路由器配置命令`traffic-share balanced`the `traffic-share balanced` router configuration command就可以开启此智能流量分配功能this intelligent traffic sharing functionality该命令无需显式配置默认是开启的**。
> **注意:** 该`traffic-share balanced`命令默认是开启的,且就算对其进行了显式配置,其也不会在运行配置中出现。这一点在下面进行了演示:
```console
R2(config)#router eigrp 150
R2(config-router)#vari 2
R2(config-router)#traffic-share balanced
R2(config-router)#exit
R2(config)#do show run | begin router
router eigrp 150
variance 2
network 150.1.1.2 0.0.0.0
network 150.2.2.2 0.0.0.0
no auto-summary
```
如同本小节前面指出的那样,在使用了`variance`命令时,所有那些满足了可行条件、且有着低于最小度量值乘以那个倍数的路径,都将被安装到路由表中。路由器随后将使用到所有路径,并依据其各自的路由度量值来按比例地进行负载均衡。
在某些情况下可能打算将那些替代路由比如可行后继路预先放入到路由表中却只在后继路由被移除时才使用它们。执行这类操作的典型目的是为在开启EIGRP的网络中降低收敛时间。要弄明白此概念就要回忆一下路由器默认是只将后继路由放入到路由表中的。而在后继路由已不可用时可行后继路由就被提升为后继路由。随后该路由才被装入到路由表中作为到目的网络的主要路径。
可将路由器配置命令`traffic-share min across-interfaces`,与`variance`命令结合使用,从而将那些有着少于最小度量值乘以所指定倍数所有路由,都安装到路由表中,却仅使用最小(最优)度量值的那条路由(后继路由),来转发数据包,直到该路由变为不可用。此配置的主要目的,就是在主路由丢失时,保证替代路由已经处于路由表中,且立即可用(从而达到减少收敛时间的目的)。
下面的配置示例使用了以上图36.11中展示的拓扑,用于对如何将路由器配置为把那些度量值少于最小度量值两倍的路由,放入到路由表中,而仅使用有着最低度量值的那条路由(后继路由)来转发数据包:
```console
R2(config)#router eigrp 150
R2(config-router)#vari 2
R2(config-router)#traffic-share min across-interfaces
R2(config-router)#exit
```
此配置造成路由表中`172.16.100.0/24`前缀的以下输出:
```console
R2#show ip route 172.16.100.0 255.255.255.0
Routing entry for 172.16.100.0/24
Known via “eigrp 150”, distance 90, metric 3014400, type internal
Redistributing via eigrp 150
Last update from 150.2.2.1 on Serial0/1, 00:09:01 ago
Routing Descriptor Blocks:
150.2.2.1, from 150.2.2.1, 00:09:01 ago, via Serial0/1
Route metric is 3847680, traffic share count is 0
Total delay is 20100 microseconds, minimum bandwidth is 768 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
* 150.1.1.1, from 150.1.1.1, 00:09:01 ago, via Serial0/0
Route metric is 3014400, traffic share count is 1
Total delay is 20100 microseconds, minimum bandwidth is 1024 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
```
如上面的输出所示,基于该种不同开销下的负载均衡配置,两条不同度量值的路由,已被安装到路由表中。但注意到经由`Serial0/1`的流量分享计数the traffic share count是0而经由`Serial0/0`的计数为1。这就意味着尽管该路由条目已被安装到路由表中, 该路由器不会通过`Serial0/1`,向`172.16.100.0/24`网络发送任何数据包,除非经由`Serial0/0`的路径不再可用。
## 使用EIGRP作默认路由
**Default Routing Using EIGRP**
在将**最终网关**或网络动态地通告给路由域中的其它路由器方面EIGRP支持多种不同方式。最终网关或默认路由是在目的网络未在路由表中特别列出时路由器用于引导流量的一种方式Enhanced IGRP supports numerous ways to advertise dynamically the gateway or network of last resort to other routers within the routing domain. **A gateway of last resort**, or default route, is a method for the router to direct traffic when the destination network is not specifically listed in the routing table。而在此种情形下路由器引导流量的方式有
- 使用`ip default-network`命令, using the `ip default-network` command
- 使用`network`命令来对网络`0.0.0.0/0`通告, using the `network` command to advertise network `0.0.0.0/0`
- 对默认静态路由进行重分发, Redistributing the default static route
- 使用命令`ip summary-address eigrp [asn] [network] [mask]`, using the `ip summary-address eigrp [asn] [network] [mask]` command
而第一种,使用`ip default-network`命令被认为是一种EIGRP下对默认路由进行动态通告的过时方式。但因为在当前的IOS软件中仍然支持这种方式所以这里有必要提到。
`ip default-network`配置命令通过把一个星号插入到路由表中某个网络旁边而将该网络标记为默认网络。那些目的地没有明确路由表条目的流量就会被路由器转发到这个网络Traffic for destinations to which there is no specific routing table entry is then forwarded by the router to this network。下面参考图36.12中的EIGRP拓扑对此种部署进行了演示
![EIGRP的默认路由](images/3612.png)
*图 36.12 -- EIGRP的默认路由*
参考图36.12, 假设子网`200.10.10.0/24`是连接到互联网的。该子网位于路由器`R1`的`Fastethernet0/0`侧。路由器`R1`与`R2`相应地通过一条背靠背的串行连接相连。两台路由器都是处于EIGRP AS 150中。为了将`200.10.10.0/24`标记为最终网络,就要在路由器`R1`上进行如下配置:
```console
R1(config)#router eigrp 150
R1(config-router)#network 200.10.10.0 0.0.0.255
R1(config-router)#exit
R1(config)#ip default-network 200.10.10.0
R1(config)#exit
```
基于此配置,路由器`R2`就会将`200.10.10.0/24`作为最终网络接收下来,如下所示:
```console
R2#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is 150.2.2.1 to network 200.10.10.0
D* 200.10.10.0/24 [90/2172416] via 150.2.2.1, 00:01:03, Serial0/0
150.1.0.0/24 is subnetted, 1 subnets
C 150.1.1.0 is directly connected, Serial0/0
```
`network`命令可用于对某条既有的指向某个物理或逻辑接口,通常是`Null0`接口的静态默认路由进行通告The `network` command can be used to advertise an existing static default route point to either a physical or a logical interface, typically the `Null0` interface
> **注意:** `Null0`接口是路由器上的一个虚拟接口,会将路由至该接口的所有流量进行抛弃处理。如有着一条指向`Null0`的静态路由,那么所有以该静态路由中所指定网络为目的的流量,都将被简单地做丢弃处理。可将`Null0`接口看着是一个黑洞数据包进入了但不会有任何东西离开那里。其基本上就是路由器上的一个数位垃圾桶It is essentially a bit-bucket on the router
参考上面的图36.12, `network`命令与一条既有默认静态路由的结合使用,在以下路由器`R1`的配置中进行了演示:
```console
R1(config)#ip route 0.0.0.0 0.0.0.0 FastEthernet0/0
R1(config)#router eigrp 150
R1(config-router)#network 0.0.0.0
R1(config-router)#exit
```
基于此种配置,下面的输出,演示了路由器`R2`上的IP路由表
```console
R2#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is 150.1.1.1 to network 0.0.0.0
D 200.10.10.0/24 [90/2172416] via 150.1.1.1, 00:01:11, Serial0/0
150.1.0.0/24 is subnetted, 1 subnets
C 150.1.1.0 is directly connected, Serial0/0
D* 0.0.0.0/0 [90/2172416] via 150.1.1.1, 00:00:43, Serial0/0
```
尽管路由重分发route redistribution不是CCNA考试部分这里仍将对其进行一个概述。路由重分发正是通过EIGRP对一条默认路由进行通告的第三种方式。为将现有的静态默认路由重分发到EIGRP中就要使用路由器配置命令`redistribute static metric [bandwidth] [delay] [reliability] [load] [MTU]`。用于本小节中前面那些输出的同一网络拓扑将用于对这种方式的部署进行演示如下图36.13所示:
![EIGRP的默认路由](images/3613.png)
*图 36.13 -- EIGRP的默认路由*
参考图36.13, 该图与图36.12是一样的,在路由器`R1`上完成以下配置:
```console
R1(config)#ip route 0.0.0.0 0.0.0.0 FastEthernet0/0
R1(config)#router eigrp 150
R1(config-router)#redistribute static metric 100000 100 255 1 1500
R1(config-router)#exit
```
> **注意:** 这里度量值中所用到的那些数值,可从接口上进行继承到,或可以在使用此命令时指定想要的任意数值。
基于此种配置,路由器`R2`上的路由表就如下所示了:
```console
R2#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is 150.1.1.1 to network 0.0.0.0
150.1.0.0/24 is subnetted, 1 subnets
C 150.1.1.0 is directly connected, Serial0/0
D*EX 0.0.0.0/0 [170/2195456] via 150.1.1.1, 00:01:16, Serial0/0
```
因为该路由是在路由器`R1`上被重分发到EIGRP中的所以如同上面所反应出的其就是一条外部EIGRP路由了。对于那些外部路由EIGRP拓扑表中就包含了诸如该路由所起源的路由器、该路由是为何种协议接收的以及该外部路由的度量值等信息。下面的输出对此进行了演示
```console
R2#show ip eigrp topology 0.0.0.0/0
IP-EIGRP (AS 150): Topology entry for 0.0.0.0/0
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2195456
Routing Descriptor Blocks:
150.1.1.1 (Serial0/0), from 150.1.1.1, Send flag is 0x0
Composite metric is (2195456/51200), Route is External
Vector metric:
Minimum bandwidth is 1544 Kbit
Total delay is 21000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
External data:
Originating router is 1.1.1.1
AS number of route is 0
External protocol is Static, external metric is 0
Administrator tag is 0 (0x00000000)
Exterior flag is set
```
可以看出该默认路由是一条在路由器`R1`上重分发到EIGRP中的静态路由。该路由有着度量值0。此外还可以看出路由器`R1`的EIGRP路由器IDthe EIGRP router ID, RID为`1.1.1.1`。
对默认路由进行通告的最后一种方法,就是使用接口配置命令`ip summary-address eigrp [asn] [network] [mask]`了。在本课程模块的后面将对EIGRP的路由汇总EIGRP route summarization进行详细讲解。这里要着重于在应用EIGRP时使用此命令来对默认路由进行通告的用途。
参考上面图36.13中所演示的网络拓扑图示,这里使用了接口配置命令`ip summary-address eigrp [asn] [network] [mask]`, 将路由器`R1`配置为把默认路由通告给`R2`,如下所示:
```console
R1(config)#interface Serial0/0
R1(config-if)#description Back-to-Back Serial Connection To R2 Serial0/0
R1(config-if)#ip summary-address eigrp 150 0.0.0.0 0.0.0.0
R1(config-if)#exit
```
使用这个命令的**主要优势在于无需为了让EIGRP将网络`0.0.0.0/0`通告给邻居路由器,而将某条默认路由或某个默认网络放入到路由表中**The primary advantage to using this command is that a default route or network does not need to exist in the routing table in order for EIGRP to advertise network `0.0.0.0/0` to its neighbour routers。在执行了此命令后本地路由器将生成一条到`Null0`接口的汇总路由并将该条目标记为备选默认路由the candidate default route。如下所示
```console
R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
150.1.0.0/24 is subnetted, 1 subnets
C 150.1.1.0 is directly connected, Serial0/0
D* 0.0.0.0/0 is a summary, 00:02:26, Null0
```
于是在路由器`R2`上就作为一条内部EIGRP路由an internal EIGRP route接收到该汇总路由如下所示
```console
R2#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is 150.1.1.1 to network 0.0.0.0
150.1.0.0/24 is subnetted, 1 subnets
C 150.1.1.0 is directly connected, Serial0/0
D* 0.0.0.0/0 [90/2297856] via 150.1.1.1, 00:03:07, Serial0/0
```
## EIGRP网络中的水平分割
**Split Horizon in EIGRP Networks**
前面已获悉水平分割是一项距离矢量协议的特性该特性强制路由信息无法从接收到的接口再发送回去这样做就阻止了路由信息重新通告到学习到其的同一接口Previously, you learned that split horizon is a Distance Vector protocol feature mandating that routing information cannot be sent back out of the same interface through which it was received. This prevents the re-advertising of information back to the source from which it was learned。虽然这个特征是一种很好的环回防止机制a great loop prevention mechanism但其也是一项明显的缺陷特别是在星形网络中especially in hub-and-spoke networks。为更好地理解此特性的不足就要参考下面图36.14中的EIGRP星形网络
![EIGRP的水平分割](images/3614.png)
*图 36.14 -- EIGRP的水平分割*
图36.14中的拓扑演示了一个典型的星形网络其中的总部路由器router HQ是中心路由器the hub router, 路由器`S1`与`S2`则是分支路由器(the spoke router)。在该帧中继广域网上每台分支路由器都有着一个在部分网状拓扑a partial-mesh topology各自与中心路由器之间的所提供的数据链路层连接标识Data Link Connection Identifier这是个6位标识表示正在进行的客户和服务器之间的连接。用于RFCOMM 层。On the Frame Relay WAN, each spoke router has a single DLCI provisioned between itself and the HQ router in a partial-mesh topology。下面对这些路由器上的帧中继配置进行了检查
```console
HQ#show frame-relay map
Serial0/0 (up): ip 172.16.1.2 dlci 102(0x66,0x1860), static,
broadcast,
CISCO, status defined, active
Serial0/0 (up): ip 172.16.1.1 dlci 103(0x67,0x1870), static,
broadcast,
CISCO, status defined, active
S1#show frame-relay map
Serial0/0 (up): ip 172.16.1.2 dlci 301(0x12D,0x48D0), static,
broadcast,
CISCO, status defined, active
Serial0/0 (up): ip 172.16.1.3 dlci 301(0x12D,0x48D0), static,
broadcast,
CISCO, status defined, active
S2#show frame-relay map
Serial0/0 (up): ip 172.16.1.1 dlci 201(0xC9,0x3090), static,
broadcast,
CISCO, status defined, active
Serial0/0 (up): ip 172.16.1.3 dlci 201(0xC9,0x3090), static,
broadcast,
CISCO, status defined, active
```
在后面的广域网章节将涉及到帧中继。这里在所有三台路由器上都开启了EIGRP使用了自治系统编号`150`。下面的输出演示了中心路由器与分支路由器之间的EIGRP邻居关系
```console
HQ#show ip eigrp neighbors
IP-EIGRP neighbors for process 150
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
1 172.16.1.1 Se0/0 165 00:01:07 24 200 0 2
0 172.16.1.2 Se0/0 153 00:01:25 124 744 0 2
```
下面的输出对第一台分支路由器`S1`与中心路由器之间的EIGRP邻居关系
```console
S1#show ip eigrp neighbors
IP-EIGRP neighbors for process 150
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 172.16.1.3 Se0/0 128 00:00:53 911 5000 0 4
```
下面的输出对第二台分支路由器`S2`与中心路由器之间的EIGRP邻居关系
```console
S2#show ip eigrp neighbors
IP-EIGRP neighbors for process 150
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 172.16.1.3 Se0/0 156 00:02:20 8 200 0 4
```
默认EIGRP的水平分割是开启的但在**局部网状网络的非广播多路访问网络**上EIGRP的水平分割是不合需要的By default, EIGRP split horizon is enabled, which is undesirable in **partial-mesh NBMA networks**)。这就意味着对于那些在`Serial0/0`上学习到的路由信息,中心路由器不会再从相同接口(`Serial0/0`)进行通告。而这种默认行为的效果,就是中心路由器不会将接收自路由器`S1`的`10.1.1.0/24`前缀,通告给`S2`,因为该路由是经由`Serial0/0`接口接收到的,而水平分割特性阻止了该路由器对从该接口学习到的信息,在通告出同一接口。这一点对于中心路由器接收自路由器`S2`的`10.2.2.0/24`前缀同样适用。
这种默认行为意味着尽管中心路由器注意到了这两条前缀,但分支路由器却只有局部的路由表。中心路由器上的路由表如下:
```console
HQ#show ip route eigrp
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 10.1.1.0/24 [90/2195456] via 172.16.1.1, 00:12:04, Serial0/0
D 10.2.2.0/24 [90/2195456] via 172.16.1.2, 00:12:06, Serial0/0
```
分支路由器`S1`上的路由表如下:
```console
S1#show ip route eigrp
192.168.1.0/26 is subnetted, 1 subnets
D 192.168.1.0 [90/2195456] via 172.16.1.3, 00:10:53, Serial0/0
```
分支路由器`S2`上的路由表如下:
```console
S2#show ip route eigrp
192.168.1.0/26 is subnetted, 1 subnets
D 192.168.1.0 [90/2195456] via 172.16.1.3, 00:10:55, Serial0/0
```
这种默认行为的结果,就是尽管中心(总部)路由器能够到达两个分支路由器的网络,但两台分支路由器却都无法到达对方的网络。解决此种情况的方法有几种,如下所示:
- 在中心路由器上关闭水平分割, Disabling split horizon on the HQ(hub) router
- 从中心路由器往分支路由器通告一条默认路由, Advertising a default route from the HQ router to the spoke routers
- 在路由器上手动配置EIGRP邻居, Mannually configuring EIGRP neighbours on the routers
通过在中心路由器的接口级别使用接口配置命令`no ip split-horizon eigrp [AS]`,就可以完成关闭水平分割。**命令`show ip split-horizon interface_name`不会显示EIGRP的水平分割状态因为该命令是作用于RIP的**。所以要查看到EIGRP的水平分割状态就必须对接口配置部分进行检查也就是执行`show run interface_name`命令。参考上面图36.14中所演示的网络拓扑,此接口配置命令就应在中心路由器上的`Serial0/0`接口上应用。应像下面这样完成:
```console
HQ(config)#interface Serial0/0
HQ(config-if)#no ip split-horizon eigrp 150
```
在水平分割关闭后,中心路由器就可以把在某个接口上接收到的路由信息,再在该接口上发送出去了。比如,分支路由器`S2`上的路由表现在就显示了一个由分支`S1`通告给中心路由器的`10.1.1.0/24`前缀了:
```console
S2#show ip route eigrp
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 10.1.1.0/24 [90/2707456] via 172.16.1.3, 00:00:47, Serial0/0
192.168.1.0/26 is subnetted, 1 subnets
D 192.168.1.0 [90/2195456] via 172.16.1.3, 00:00:47, Serial0/0
```
可使用一个简单的从分支路由器`S2`到`10.1.1.0/24`的`ping`操作,对连通性进行检查,如下所示:
```console
S2#ping 10.1.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 24/27/32 ms
```
关闭水平分割的第二种方法,就是简单地从中心路由器往分支路由器通告一条默认路由。在这种情况下,可以将接口配置命令`ip summary-address eigrp 150 0.0.0.0 0.0.0.0`,应用到中心路由器的`Serial0/0`接口。这样做就令到分支路由器能够经由中心路由器,到达对方的网络,中心路由器包含了完整的路由表,从而消除了对关闭水平分割的需求。
而关闭水平分割的第三种替代方式,就是在**所有路由器**上,使用路由器配置命令`neighbor`来手动配置一些EIGRP的邻居语句。因为在使用了此种配置后邻居间的更新就是单播的了所以水平分割的限制条件就得以消除。此选项在小型网络中用起来是不错的但随着网络的增大以及分支路由器数量的增加工作量也会增加。
因为EIGRP默认路由与静态邻居方面的配置在本课程模块的前面小节都已详细介绍过了所以这里为了简洁期间这些特性的配置就做了省略处理。
## EIGRP的路由汇总
**EIGRP Route Summarisation**
路由汇总特性减少了路由器必须处理的信息量,这样就实现了网络更快的收敛。通过将网络中具体区域的详细拓扑信息的隐藏,汇总还对由网络变动而造成的影响范围有所限制。最后,如同本课程模块前面所指出的那样,汇总还用于**EIGRP查询边界**的定义,而**EIGRP查询边界又支持两种类型的路由汇总**Route summarisation reduces the amount of information that routers must process, which allows for faster convergence within the network. Summarisation also restricts the size of the area that is affected by network changes by hiding detailed topology information from certain areas within the network. Finally, as was stated earlier in this module, summarisation is used to define **a Query boundary for EIGRP, which supports two types of route summarisation**),如下所示:
- 自动的路由汇总, Automatic route summarisation
- 手动的路由汇总, Manual route summarisation
默认情况下当在路由器上开启了EIGRP时自动路由汇总就生效了。这是通过使用`auto-summary`命令应用的。该命令允许EIGRP在**有类边界**at classful boundaries进行自动路由汇总。参考下图36.15中的网络拓扑,对此默认特性的运作进行了演示:
![EIGRP的自动路由汇总](images/3615.png)
*图 36.15 -- EIGRP的自动路由汇总*
参考图36.15中的EIGRP网络路由器`R1`与`R2`运行着EIGRP使用着自治系统编号150。`10.1.1.0/24`、`10.2.2.0/24`与`10.3.3.3.0/24`子网都是直接连接到路由器`R1`上的。路由器`R1`正将这些路由通告给`R2`。路由器`R1`与`R2`是使用了一条在`150.1.1.0/24`子网(该子网是一个与`10.1.1.0/24`、`10.2.2.0/24`及`10.3.3.3.0/24`子网都不相同的主要网络(major network), 上的背靠背的串行链路连接起来的。根据连接在这些路由器上的网络默认EIGRP将执行自动汇总如下所示
- `10.1.1.0/24`、`10.2.2.0/24`与`10.3.3.3.0/24`子网将被汇总到`10.0.0.0/8`
- `150.1.1.0/24`子网将被汇总到`150.1.0.0/16`
可以查看`show ip protocols`命令的输出,来对此默认行为进行验证。路由器`R1`上该命令的输出如下所示:
```console
R1#show ip protocols
Routing Protocol is “eigrp 150”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 150
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Automatic address summarization:
150.1.0.0/16 for Loopback1, Loopback2, Loopback3
Summarizing with metric 2169856
10.0.0.0/8 for Serial0/0
Summarizing with metric 128256
Maximum path: 4
Routing for Networks:
10.1.1.0/24
10.2.2.0/24
10.3.3.0/24
150.1.1.0/24
Routing Information Sources:
Gateway Distance Last Update
(this router) 90 00:03:12
150.1.1.2 90 00:03:12
Distance: internal 90 external 170
```
在上面的输出中,`10.1.1.0/24`、`10.2.2.0/24`与`10.3.3.3.0/24`子网已被自动汇总到`10.0.0.0/8`。该汇总地址就被通告出`Serial0/0`接口。而`150.1.1.0/24`子网已被汇总到`150.1.0.0/16`。该汇总地址就被通告出`Loopback1`、`Loopback2`与`Loopback3`这三个环回接口。这里要记住默认EIGRP将在所有EIGRP路由开启的接口上发出路由更新Remember, by default, EIGRP will send out updates on all interfaces for which EIGRP routing is enabled
参考上面的打印输出可以看到环回接口上所发出的更新就是一种资源的浪费因为设备是无法物理连接到路由器环回接口上去监听此类更新的Referencing the output printed above, you can see that sending updates on a Loopback interface is a waste of resource because a device cannot be connected physically to a router Loopback interface listening for such updates。那么就可以通告使用路由器配置命令`passive-interface`,来关闭此默认行为,如下所示:
```console
R1(config)#router eigrp 150
R1(config-router)#passive-interface Loopback1
R1(config-router)#passive-interface Loopback2
R1(config-router)#passive-interface Loopback3
R1(config-router)#exit
```
这样的配置的结果就是不再往这些环回发出EIGRP数据包了。因此如下所示汇总地址就不会在这些接口上通告出去了
```console
R1#show ip protocols
Routing Protocol is “eigrp 150”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 150
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Automatic address summarization:
10.0.0.0/8 for Serial0/0
Summarizing with metric 128256
Maximum path: 4
Routing for Networks:
10.0.0.0
150.1.0.0
Passive Interface(s):
Loopback0
Loopback1
Loopback2
Loopback3
Routing Information Sources:
Gateway Distance Last Update
(this router) 90 00:03:07
150.1.1.2 90 00:01:12
Distance: internal 90 external 170
```
> **注意:** 本课程模块稍后会对`passive-interface`命令进行详细讲解。
继续有关自动汇总方面的内容在对有类边界进行自动汇总后EIGRP就将一条到汇总地址的路由安装到EIGRP的拓扑表与IP路由表中Continuing with automatic summarisation, following automatic summarisation at the classful boundary, EIGRP installs a route to the summary address into the EIGRP topology table and the IP routing table。下面的EIGRP拓扑表中就包含了此汇总地址的路由以及更具体的路由条目以及这些路由条目各自所直接连接的接口
```console
R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(150)/ID(10.3.3.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.0.0.0/8, 1 successors, FD is 128256
via Summary (128256/0), Null0
P 10.3.3.0/24, 1 successors, FD is 128256
via Connected, Loopback3
P 10.2.2.0/24, 1 successors, FD is 128256
via Connected, Loopback2
P 10.1.1.0/24, 1 successors, FD is 128256
via Connected, Loopback1
...
[Truncated Output]
```
在路由表中,汇总路由是直接连接到`Null0`接口上的。该路由有着一个默认为`5`的管理距离数值a default administrative distance value of `5`)。下面的输出对此进行了演示:
```console
R1#show ip route 10.0.0.0 255.0.0.0
Routing entry for 10.0.0.0/8
Known via “eigrp 150”, distance 5, metric 128256, type internal
Redistributing via eigrp 150
Routing Descriptor Blocks:
* directly connected, via Null0
Route metric is 128256, traffic share count is 1
Total delay is 5000 microseconds, minimum bandwidth is 10000000 Kbit
Reliability 255/255, minimum MTU 1514 bytes
Loading 1/255, Hops 0
```
在EIGRP完成自动汇总时路由器将对汇总路由进行通告从而抑制了那些更为具体路由的通告When EIGRP performs automatic summarisation, the router advertises the summary route and suppresses the more specific routes。换句话说在汇总路由得以通告时那些更为具体的前缀在发往EIGRP邻居的更新中就被省略了the more specific prefixes are suppressed in updates to EIGRP neighbours。这一点可通过查看路由器`R2`上的路由表,加以验证,如下所示:
```console
R2#show ip route eigrp
D 10.0.0.0/8 [90/2298856] via 150.1.1.1, 00:29:05, Serial0/0
```
这种默认行为在一些基本的网络比如上图36.15中演示的网络中,运作不错。但其可能在**不连续网络**--那种由两个分离的主要网络所组成的网络中如下图36.16中所演示的有着不利影响However, it can have an adverse impact in **a discontiguous network**, which comprises a major network that separates another major network
![不连续网络](images/3616.png)
*图 36.16 -- 不连续网络, Discontiguous Network*
参考图36.16中所演示的图例,一个大的`150.1.0.0/16`网络将这里的两个大的`10.0.0.0/8`网络分开了。在开启自动汇总时,路由器`R1`与`R2`将把`10.1.1.0/24`及`10.2.2.0/24`子网,相应地都汇总到`10.0.0.0/8`地址。该汇总路由将以下一跳接口`Null0`,被安装(到路由表中)。而`Null0`接口又是一个“数位垃圾桶(bit-bucket)”。所有发送到此接口的数据包,就将实实在在地被丢弃。
> **译者注**本小节及前面小节中所提到的大网络the major network是指按网络大类分的网络也就是A、B、C、D及E类网络。
因为两台路由器就只把汇总地址通告给对方,所以两台路由器都将无法到达对方的`10.x.x.x/24`子网。为掌握到图36.16所演示网络中自动汇总的衍生问题ramifications下面从在路由器`R1`及`R2`上的配置开始,一次一步的走一下这些步骤,如下所示:
```console
R1(config)#router eigrp 150
R1(config-router)#network 10.1.1.0 0.0.0.255
R1(config-router)#network 150.1.1.0 0.0.0.255
R1(config-router)#exit
```
```console
R2(config)#router eigrp 150
R2(config-router)#network 10.2.2.0 0.0.0.255
R2(config-router)#network 150.1.1.0 0.0.0.255
R2(config-router)#exit
```
因为两台路由器上有类边界处的自动汇总都是默认开启的,所以这两台路由器都将生成两个汇总地址:一个是`10.0.0.0/8`的,另一个是`150.1.0.0/16`的Because automatic summarisation at the classful boundary is enabled by default on both of the routers, they will both generate two summary addresses: one for `10.0.0.0/8` and another for `150.1.0.0/16`)。这两个汇总地址都将指向到`Null0`接口,同时路由器`R1`上的路由表将显示下面这些条目:
```console
R1#show ip route eigrp
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 10.0.0.0/8 is a summary, 00:04:51, Null0
150.1.0.0/16 is variably subnetted, 2 subnets, 2 masks
D 150.1.0.0/16 is a summary, 00:06:22, Null0
```
与此类似,路由器`R2`上反应了同样的情况,如下所示:
```console
R2#show ip route eigrp
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 10.0.0.0/8 is a summary, 00:01:58, Null0
150.1.0.0/16 is variably subnetted, 2 subnets, 2 masks
D 150.1.0.0/16 is a summary, 00:01:58, Null0
```
虽然汇总地址`150.1.0.0/16`被安装到了IP路由表中路由器`R1`与`R2`是仍可`ping`通对方的因为这里的更为具体路由条目the more route-specific entry, `150.1.1.0/24`)是位处于直连接口之上的。可通过执行`show ip route [address] [mask] longer-prefixes`命令,来查看到这些某个汇总路由中的详细条目。下面演示了对于`150.1.0.0/16`汇总,该命令的输出:
```console
R1#show ip route 150.1.0.0 255.255.0.0 longer-prefixes
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
150.1.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 150.1.1.0/24 is directly connected, Serial0/0
D 150.1.0.0/16 is a summary, 00:10:29, Null0
```
因为那条更详细的`150.1.1.0/24`路由条目是存在的,所以发送到`150.1.1.2`地址的数据包将经由`Serial0/0`接口加以转发。这就令到路由器`R1`与`R2`的连通性没有问题,如下所示:
```console
R1#ping 150.1.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 150.1.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms
```
但是,那些到这个大的`150.1.0.0/16`网络的所有其它子网的数据包,都将发送到`Null0`接口,因为没有其它具体路由条目存在(于路由表中)。
那么到这里一切都说得通了So far, everything appears to be in order。可以看出因为大的`150.1.0.0/16`网络的这条更具体路由条目(`150.1.1.0/24`)的存在,路由器`R1`与`R2`之间是能`ping`通的。不过问题在于路由器`R1`与`R2`上大的网络`10.0.0.0/8`的那些子网。路由器`R1`(的路由表)显示出下面其生成的`10.0.0.0/8`汇总地址的具体路由条目:
```console
R1#show ip route 10.0.0.0 255.0.0.0 longer-prefixes
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.1.1.0/24 is directly connected, FastEthernet0/0
D 10.0.0.0/8 is a summary, 00:14:23, Null0
```
于此类似,路由器`R2`(的路由表)显示出其生成的`10.0.0.0/8`汇总地址的以下具体路由条目:
```console
R2#show ip route 10.0.0.0 255.0.0.0 longer-prefixes
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.2.2.0/24 is directly connected, FastEthernet0/0
D 10.0.0.0/8 is a summary, 00:14:23, Null0
```
可以看出,两台路由器都没有到对方的`10.x.x.x/24`子网的路由。假设在路由器`R1`尝试往`10.2.2.0/24`发送数据包时,将使用那个汇总地址,那么数据包就将转发到`Null0`接口。下面的输出对此进行了演示:
```console
R1#show ip route 10.2.2.0
Routing entry for 10.0.0.0/8
Known via “eigrp 150”, distance 5, metric 28160, type internal
Redistributing via eigrp 150
Routing Descriptor Blocks:
* directly connected, via Null0
Route metric is 28160, traffic share count is 1
Total delay is 100 microseconds, minimum bandwidth is 100000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 0
```
路由器`R1`将无法`ping`通`R2`上的`10.x.x.x/24`子网,反之亦然,如下所示:
```console
R1#ping 10.2.2.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.2.2.2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
```
而要解决这个问题,则有两种方案,如下:
- 在两台路由器上手动配置`10.x.x.x/24`的静态路由, Mannually configure static routes for the `10.x.x.x/24` subnets on both routers
- 关闭EIGRP的自动有类网络汇总Disable EIGRP automatic classful network summarisation
第一个选项是相当简单粗暴基础的very basic。但是静态路由配置不具备可伸缩性且在大型网络中需要大量费时费力的配置。而第二选项也就是**推荐做法**,除了具备伸缩性,比起第一选项只需较少的时间精力。**通过执行`no auto-summary`命令**,就可将自动汇总予以关闭(**较新版本的IOS中该特性已被默认关闭了**, 如下所示:
```console
R1(config)#router eigrp 150
R1(config-router)#no auto-summary
R1(config-router)#exit
```
```console
R2(config)#router eigrp 150
R2(config-router)#no auto-summary
R2(config-router)#exit
```
此配置的结果,就是大的网络上的那些具体子网,在两台路由器上都有通告。不会生成汇总路由,如下所示:
```console
R2#show ip route eigrp
10.0.0.0/24 is subnetted, 2 subnets
D 10.1.1.0 [90/2172416] via 150.1.1.1, 00:01:17, Serial0/0
```
这些`10.x.x.x/24`子网之间的连通性,可使用一个简单的`ping`操作,加以验证,如下所示:
```console
R2#ping 10.1.1.1 source 10.2.2.2 repeat 10
Type escape sequence to abort.
Sending 10, 100-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:
Packet sent with a source address of 10.2.2.2
!!!!!!!!!!
Success rate is 100 percent (10/10), round-trip min/avg/max = 1/3/4 ms
```
在深入讨论手动路由汇总前重要的是先要知道EIGRP是不会自动对外部网络进行汇总的, 除非某个内部网络将包含在那个汇总中Before we go into the details pertaining to manual summarisation, it is important to know that EIGRP will not automaticlly summarise external networks unless there is an internal network that will be included in the summary。为更好地掌握此概念就要参考下图36.17, 该图片演示了一个基本的EIGRP网络
![对外部网络的汇总](images/3617.png)
*图 36.17 -- 对外部网络的汇总Summrising External Networks*
参考图36.17, 路由器`R1`正在进行重分发redistributing, 这令到这些网络成为外部的网络),并在随后将外部网络`10.0.0.0/24`、`10.1.1.0/24`、`10.2.2.0/24`与`10.3.3.0/24`这些外部网络经由EIGRP进行通告。路由器`R1`上开启了自动路由汇总。路由器`R1`上的初始配置是下面这样的:
```console
R1(config)#router eigrp 150
R1(config-router)#redistribute connected metric 8000000 5000 255 1 1514
R1(config-router)#network 150.1.1.1 0.0.0.0
R1(config-router)#exit
```
`show ip protocols`命令显示出路由器`R1`的`Serial0/0`接口上开启了EIGRP, 同时正通告着那些连接的网络。同时自动汇总是开启的,如下所示:
```console
R1#show ip protocols
Routing Protocol is “eigrp 150”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: connected, eigrp 150
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Maximum path: 4
Routing for Networks:
150.1.1.1/32
Routing Information Sources:
Gateway Distance Last Update
150.1.1.2 90 00:00:07
Distance: internal 90 external 170
```
就像前面输出示例所演示的那样, 因为这些`10.x.x.x/24`前缀都是外部路由所以EIGRP不会对这些前缀进行自动汇总。又因此EIGRP不会往拓扑表也不会往IP路由表中添加一条这些前缀的汇总路由。下面的输出对此进行了演示
```console
R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(150)/ID(10.3.3.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.0.0.0/24, 1 successors, FD is 1280256
via Rconnected (1280256/0)
P 10.1.1.0/24, 1 successors, FD is 1280256
via Rconnected (1280256/0)
P 10.2.2.0/24, 1 successors, FD is 1280256
via Rconnected (1280256/0)
P 10.3.3.0/24, 1 successors, FD is 1280256
via Rconnected (1280256/0)
...
[Truncated Output]
```
这些具体路由条目是作为外部EIGRP路由通告给路由器`R2`的,如下所示:
```console
R2#show ip route eigrp
10.0.0.0/24 is subnetted, 4 subnets
D EX 10.3.3.0 [170/3449856] via 150.1.1.1, 00:07:02, Serial0/0
D EX 10.2.2.0 [170/3449856] via 150.1.1.1, 00:07:02, Serial0/0
D EX 10.1.1.0 [170/3449856] via 150.1.1.1, 00:07:02, Serial0/0
D EX 10.0.0.0 [170/3449856] via 150.1.1.1, 00:07:02, Serial0/0
```
现在,假设`10.0.0.0/24`是一个内部网络,而`10.1.1.0/24`、`10.2.2.0/24`与`10.3.3.0/24`三个子网却是外部网络。因为这些将构成有类汇总地址`10.0.0.0/8`的路由中有一条是内部路由所以EIGRP将创建出一个汇总地址并将其包含在EIGRP拓扑表与IP路由表中。命令`show ip protocols`显示出`10.0.0.0/24`网络此时就是一个内部EIGRP网络了如下所示
```console
R1#show ip protocols
Routing Protocol is “eigrp 150”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: connected, eigrp 150
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Automatic address summarization:
150.1.0.0/16 for Loopback0
Summarizing with metric 2169856
10.0.0.0/8 for Serial0/0
Summarizing with metric 128256
Maximum path: 4
Routing for Networks:
10.0.0.1/32
150.1.1.1/32
Routing Information Sources:
Gateway Distance Last Update
(this router) 90 00:00:05
150.1.1.2 90 00:00:02
Distance: internal 90 external 170
```
在上面的输出中EIGRP的自动汇总已经生成一个`10.0.0.0/8`的汇总地址,因为`10.0.0.0/24`这个内部子网是该聚合地址the aggregate address的一部分。而EIGRP的拓扑表将显示出这些外部与内部条目以及生成的汇总地址如下所示
```console
R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(150)/ID(10.3.3.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.0.0.0/8, 1 successors, FD is 128256
via Summary (128256/0), Null0
P 10.0.0.0/24, 1 successors, FD is 128256
via Connected, Loopback0
P 10.1.1.0/24, 1 successors, FD is 1280256
via Rconnected (1280256/0)
P 10.2.2.0/24, 1 successors, FD is 1280256
via Rconnected (1280256/0)
P 10.3.3.0/24, 1 successors, FD is 1280256
via Rconnected (1280256/0)
...
[Truncated Output]
```
此时,就仅有一条路由通告给路由器`R2`了,如下面的输出所示:
```console
R2#show ip route eigrp
D 10.0.0.0/8 [90/2297856] via 150.1.1.1, 00:04:05, Serial0/0
```
从路由器`R2`的角度看该路由就是一条简单的内部EIGRP路由。也就是说路由器`R2`并不知道这个汇总地址还有着一些外部路由,如下所示:
```console
R2#show ip route 10.0.0.0 255.0.0.0
Routing entry for 10.0.0.0/8
Known via “eigrp 150”, distance 90, metric 2297856, type internal
Redistributing via eigrp 150
Last update from 150.1.1.1 on Serial0/0, 00:05:34 ago
Routing Descriptor Blocks:
* 150.1.1.1, from 150.1.1.1, 00:05:34 ago, via Serial0/0
Route metric is 2297856, traffic share count is 1
Total delay is 25000 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
```
经由所接收到的这条汇总路由,路由器`R2`是可以同时到达这个内部的`10.0.0.0/24`与那些其它的外部`10.x.x.x/24`网络,如下所示:
```console
R2#ping 10.0.0.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.0.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
R2#ping 10.3.3.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.3.3.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms
```
与EIGRP的自动汇总不同**EIGRP的手动路由汇总是在接口级别使用接口配置命令`ip summary-address eigrp [ASN] [network] [mask] [distance] [leak-map <name>]`,进行配置和部署的**。默认情况下分配给EIGRP汇总地址的默认**管理距离**值为`5`By default, an EIGRP summary address is assigned a default **administrative distance** value of `5`)。可通过由`[distance]`关键字所指定的数值,从而指定所需的管理距离值,来改变此默认分配的值。
默认在配置了手动路由汇总时EIGRP将不会就包含在已汇总网络条目中的那些更具体路由条目进行通告了。可将关键字`[leak-map <name>]`配置为允许EIGRP路由泄漏有了此特性EIGRP就允许那些指定的具体路由条目与汇总地址一起得以通告。而那些未在泄露图谱中指定的条目则仍然会被压减By default, when manual route summarisation is configured, EIGRP will not advertise the more specific route entries that fall within the summarised network entry. The `[leak-map <name>]` keyword can be configured to allow EIGRP route leaking, wherein EIGRP allows specified specific route entries to be advertised in conjunction with the summary address. Those entries that are not specified in the leak map are still suppressed
在对路由进行手动汇总时重要的是要足够具体When mannually summarising routes, it is important to be as specific as possible。否则所作出的配置将导致与先前所讲到的不连续网络示例中类似的流量黑洞问题。下图36.18中对此概念进行了演示:
![不良路由汇总下的流量黑洞问题](images/3618.png)
*图 36.18 -- 不良路由汇总下的流量黑洞问题Black-Holing traffic with Poor Route Summarisation*
参考图36.18, 假如在两台路由器上都手动配置了一个`10.0.0.0/8`的汇总地址那么更为具体的那些前缀就被压减掉了。因为同时EIGRP将一条到该汇总地址的路由与下一跳接口`Null0`安装到EIGRP的拓扑表及IP路由表那么在此网络中将经历与不连续网络中自动汇总同样的问题两台路由器上相应子网将无法与对方联系上。
此外同样重要的是需要明白如网络中有着不良部署手动汇总还将导致网络的次优路由问题suboptimal routing。下图36.19对此进行了演示:
![路由汇总中的次优路由问题](images/3619.png)
*图 36.19 -- 路由汇总带来的次优路由问题Suboptimal Routing with Route Summarisation*
默认情况下在EIGRP的一条汇总路由创建出来后路由器就将该汇总地址以其中所有更具体路由度量值中最小的度量值进行通告。也就是说汇总地址将有着包含在汇总地址建立中最低的、最具体路由的度量值By default, when a summary route is created for EIGRP, the router advertises the summary address with a metric equal to the minimum of all the more specific routes. In other words, the summary address will have the same metric as the lowest, most specific route included in the creation of the summary address
参考图36.19中所演示的网络拓扑,路由器`R2`与`R3`都正将汇总地址`10.0.0.0/8`通告给`R1`。此汇总是有更具体的`10.4.4.0/24`与`10.6.6.0/24`前缀构成的。该汇总地址所使用的度量值在两台路由器上分别如下表36.5那样计算出来:
*表 36.5 -- 汇总路由的度量值计算*
| 起始点(路由器) | 到`10.4.4.0/24`的度量值 | 到`10.6.6.0/24`的度量值 |
| ----- | -----: | -----: |
| `R2` | `5+5=10` | `5+45+5+5=60` |
| `R3` | `5+45+5=55` | `5+5+5=15` |
基于表36.5中的度量值计算, 对于从路由器`R1`起始的流量,`R2`显然有着到`10.4.4.0`的最低度量值路径,同时`R3`有着对于起始自`R1`、到`10.6.6.0/24`网络流量的最低度量值路径。但在汇总地址`10.0.0.0/8`被通告到路由器`R1`时,该汇总地址使用了构成该汇总路由的所有路由中最低最小度量值。在这个示例中,路由器`R2`以度量值`10`, 将该汇总地址通告给`R1`。`R3`以同样逻辑,以`15`的度量值将该汇总路由通告给`R1`。
在路由器`R1`从`R2`及`R3`处接收到两条汇总路由后,它将使用最低的度量值来转发那些以包含在大的(主要)有类网络`10.0.0.0/8`中的那些子网为目的的流量。下图36.20对此进行了演示:
![路由汇总下的次优路由](images/3620.png)
*图 36.20 -- 路由汇总下的次优路由Suboptimal Routing with Route Summarisation*
参考图36.20, 可清楚地发现,尽管这对于`10.4.4.0/24`子网来说,是一条最优路径,而对于`10.6.6.0/24`子网却是一条次优路径。因此,**在网络中部署路由汇总时,先弄清楚网络拓扑是非常重要的**。
回到采用EIGRP时的手动路由汇总的配置上下图36.21所演示的网络拓扑,将用于对手动路由汇总及**路由泄露特性**route leaking的演示
![EIGRP手动路由汇总的配置](images/3621.png)
*图 36.21 -- EIGRP手动路由汇总的配置Configuring EIGRP Manual Route Summarisation*
基于路由器`R1`上所配置的接口,`R2`上的路由表将显示出以下条目:
```console
R2#show ip route eigrp
10.0.0.0/24 is subnetted, 4 subnets
D 10.3.3.0 [90/2297856] via 150.1.1.1, 00:00:14, Serial0/0
D 10.2.2.0 [90/2297856] via 150.1.1.1, 00:00:14, Serial0/0
D 10.1.1.0 [90/2297856] via 150.1.1.1, 00:00:14, Serial0/0
D 10.0.0.0 [90/2297856] via 150.1.1.1, 00:00:14, Serial0/0
```
为将路由器`R1`上的这些路由条目进行汇总并通告出一条单一的特定(译者注,这里可能是“汇总”,而不是具体(specific))路由,就要在`R1`的`Serial0/0`接口上应用如下配置:
```console
R1(config)#interface Serial0/0
R1(config-if)#ip summary-address eigrp 150 10.0.0.0 255.252.0.0
R1(config-if)#exit
```
在此配置下,该汇总条目`10.0.0.0/14`就将被安装到路由器`R1`的EIGRP拓扑表与IP路由表中。该EIGRP拓扑表条目如下所示
```console
R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(150)/ID(10.3.3.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.0.0.0/14, 1 successors, FD is 128256
via Summary (128256/0), Null0
P 10.3.3.0/24, 1 successors, FD is 128256
via Connected, Loopback3
P 10.2.2.0/24, 1 successors, FD is 128256
via Connected, Loopback2
P 10.0.0.0/24, 1 successors, FD is 128256
via Connected, Loopback0
P 10.1.1.0/24, 1 successors, FD is 128256
via Connected, Loopback1
...
[Truncated Output]
```
而该路由表条目,也同样反应出来此汇总路由的下一跳接口为`Null0`, 如下所示:
```console
R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
C 10.3.3.0/24 is directly connected, Loopback3
C 10.2.2.0/24 is directly connected, Loopback2
C 10.1.1.0/24 is directly connected, Loopback1
C 10.0.0.0/24 is directly connected, Loopback0
D* 10.0.0.0/14 is a summary, 00:02:37, Null0
150.1.0.0/24 is subnetted, 1 subnets
C 150.1.1.0 is directly connected, Serial0/0
150.2.0.0/24 is subnetted, 1 subnets
C 150.2.2.0 is directly connected, Serial0/1
```
可再次使用`show ip route [address] [mask] longer-prefixes`命令,来查看组成该聚合或者说汇总路由的那些具体路由条目,如下面路由器`R1`上的输出所示:
```console
R1#show ip route 10.0.0.0 255.252.0.0 longer-prefixes
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
C 10.3.3.0/24 is directly connected, Loopback3
C 10.2.2.0/24 is directly connected, Loopback2
C 10.1.1.0/24 is directly connected, Loopback1
C 10.0.0.0/24 is directly connected, Loopback0
D 10.0.0.0/14 is a summary, 00:04:03, Null0
```
而在路由器`R2`上,则接收到汇总地址`10.0.0.0/14`的一条单一的路由条目,如下所示:
```console
R2#show ip route eigrp
10.0.0.0/14 is subnetted, 1 subnets
D 10.0.0.0 [90/2297856] via 150.1.1.1, 00:06:22, Serial0/0
```
为加强该汇总路由度量值概念,这里假设路由器`R1`上的那些路由都是有着不同度量值的外部路由也就是说这些路由是已被重分发到EIGRP中的。那么`R1`上的EIGRP拓扑表将显示以下条目
```console
R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(150)/ID(10.3.3.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.0.0.0/24, 1 successors, FD is 10127872
via Rconnected (10127872/0)
P 10.1.1.0/24, 1 successors, FD is 3461120
via Rconnected (3461120/0)
P 10.2.2.0/24, 1 successors, FD is 2627840
via Rconnected (2627840/0)
P 10.3.3.0/24, 1 successors, FD is 1377792
via Rconnected (1377792/0)
...
[Truncated Output]
```
此时再度在路由器`R1`上做先前示例中的相同汇总地址配置,如下所示:
```console
R1(config)#int s0/0
R1(config-if)#ip summary-address eigrp 150 10.0.0.0 255.252.0.0
R1(config-if)#exit
```
那么基于此配置该汇总路由就以它所包含的所有路由中的最小度量值而放入到EIGRP拓扑表和IP路由表中Based on this configuration, the summary route is placed into the EIGRP topology table and the IP routing table with a metric equal to the lowest metric of all routes that it encompasses。而根据前面所展示的`show ip eigrp topology`命令的输出,该汇总地址将获得到与`10.3.3.0/24`前缀相同的度量值,如下所示:
```console
R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(150)/ID(10.3.3.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.0.0.0/14, 1 successors, FD is 1377792
via Summary (1377792/0), Null0
P 10.0.0.0/24, 1 successors, FD is 10127872
via Rconnected (10127872/0)
P 10.1.1.0/24, 1 successors, FD is 3461120
via Rconnected (3461120/0)
P 10.2.2.0/24, 1 successors, FD is 2627840
via Rconnected (2627840/0)
P 10.3.3.0/24, 1 successors, FD is 1377792
via Rconnected (1377792/0)
P 150.1.1.0/24, 1 successors, FD is 2169856
via Connected, Serial0/0
```
## 什么是被动接口
**Understanding Passive Interface**
如同本课程模块前面所指出的在对某个网络开启了EIGRP后路由器就开始在其位于某个特定网络范围内的所有接口上发出Hello数据包。这样做可令到EIGRP能够动态地发现邻居并建立各种网络关系。这对于那些切实有着到物理介质连接的接口上比如以太网及串行接口等是需要的。但这种默认行为在那些绝不会有其它设备连接的逻辑接口上logical interfaces比如环回接口却是不必要的因为路由器绝不会经由这些逻辑接口建立EIGRP的邻居关系相反还会造成不必要的路由器资源浪费。
思科IOS软件允许管理员使用**路由器配置命令`passive-interface [name|default]`**, 将命名的接口the named interface或把所有接口指定为被动模式。从而不会在这些被动接口上发出EIGRP数据包; 在这些被动接口之间就绝对不会建立邻居关系了。下面的输出演示了在路由器上如何将两个开启了EIGRP的接口配置为被动模式
```console
R1(config)#interface Loopback0
R1(config-if)#ip address 10.0.0.1 255.255.255.0
R1(config-if)#exit
R1(config)#interface Loopback1
R1(config-if)#ip address 10.1.1.1 255.255.255.0
R1(config-if)#exit
R1(config)#interface Serial0/0
R1(config-if)#ip address 150.1.1.1 255.255.255.0
R1(config-if)#exit
R1(config)#router eigrp 150
R1(config-router)#no auto-summary
R1(config-router)#network 150.1.1.0 0.0.0.255
R1(config-router)#network 10.0.0.0 0.0.0.255
R1(config-router)#network 10.1.1.0 0.0.0.255
R1(config-router)#passive-interface Loopback0
R1(config-router)#passive-interface Loopback1
R1(config-router)#exit
```
基于此种配置,逻辑接口`Loopback0`与`Loopback1`是开启了EIGRP路由的它们上面的直连网络将被通告给EIGRP邻居。但路由器`R1`是不会将EIGRP数据包从这两个接口发出去的。另一方面接口`Serial0/0`上也配置上了EIGRP路由不过这里是允许EIGRP在此接口上发出数据包的因为它不是一个被动接口。所有三个网络都是安装到EIGRP路由表中的如下所示
```console
R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(150)/ID(10.3.3.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.1.1.0/24, 1 successors, FD is 128256
via Connected, Loopback1
P 10.0.0.0/24, 1 successors, FD is 128256
via Connected, Loopback0
P 150.1.1.0/24, 1 successors, FD is 2169856
via Connected, Serial0/0
```
但`show ip eigrp interfaces`显示EIGRP路由只是对`Serial0/0`接口是开启的,如下所示:
```console
R1#show ip eigrp interfaces
IP-EIGRP interfaces for process 150
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Se0/0 1 0/0 0 0/15 0 0
```
也可在`show ip protocols`命令的输出中查看到那些配置为被动模式的接口,如下所示:
```console
R1#show ip protocols
Routing Protocol is “eigrp 150”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 150
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is not in effect
Maximum path: 4
Routing for Networks:
10.0.0.0/24
10.1.1.0/24
150.1.1.0/24
Passive Interface(s):
Loopback0
Loopback1
Routing Information Sources:
Gateway Distance Last Update
Distance: internal 90 external 170
```
路由器配置命令`passive-interface [name|default]`中的`[default]`关键字令到所有接口都成为被动模式。先假设路由器上配置了50个的环回接口。如打算将每个环回接口都配置为被动模式那么就需要50行的代码。而这时就可以使用`passive-interface default`命令来令到所有接口都成为被动接口了。对于那些确实想要发出EIGRP数据包的接口就可使用`no passive-interface [name]`命令进行配置。下面对`passive-interface default`命令的用法进行了演示:
```console
R1(config)#interface Loopback0
R1(config-if)#ip address 10.0.0.1 255.255.255.0
R1(config-if)#exit
R1(config)#interface Loopback1
R1(config-if)#ip address 10.1.1.1 255.255.255.0
R1(config-if)#exit
R1(config)#interface Loopback3
R1(config-if)#ip address 10.3.3.1 255.255.255.0
R1(config-if)#exit
R1(config)#interface Loopback2
R1(config-if)#ip address 10.2.2.1 255.255.255.0
R1(config-if)#exit
R1(config)#interface Serial0/0
R1(config-if)#ip address 150.1.1.1 255.255.255.0
R1(config-if)#exit
R1(config-router)#network 10.0.0.1 255.255.255.0
R1(config-router)#network 10.1.1.1 255.255.255.0
R1(config-router)#network 10.3.3.1 255.255.255.0
R1(config-router)#network 10.2.2.1 255.255.255.0
R1(config-router)#network 150.1.1.1 255.255.255.0
R1(config-router)#passive-interface default
R1(config-router)#no passive-interface Serial0/0
R1(config-router)#exit
```
这里可以再度使用`show ip protocols`命令来查看哪些接口是处于EIGRP下的被动模式的如下所示
```console
R1#show ip protocols
Routing Protocol is “eigrp 150”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 150
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is not in effect
Maximum path: 4
Routing for Networks:
10.0.0.0/24
10.1.1.0/24
10.2.2.0/24
10.3.3.0/24
150.1.1.0/24
Passive Interface(s):
Loopback1
Loopback2
Loopback3
Loopback4
Routing Information Sources:
Gateway Distance Last Update
(this router) 90 00:02:52
Distance: internal 90 external 170
```
这里通过使用`passive-interface default`命令,令到多个被动接口的配置就得以简化,并减少了代码。而当其与`no passive-interface Serial0/0`一起使用时EIGRP数据包仍旧在接口`Serial0/0`上发出从而允许EIGRP邻居关系通过此接口建立起来如下所示
```console
R1#show ip eigrp neighbors
IP-EIGRP neighbors for process 150
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 150.1.1.2 Se0/0 12 00:02:47 1 3000 0 69
```
## 掌握EIGRP路由器ID的用法
**Understanding the Use of the EIGRP Router ID**
与OSPF使用路由器IDthe router ID, RID来识别OSPF邻居不同**EIGRP的RID主要用途是阻止路由环回的形成**。RID被**用于识别那些外部路由的起源路由器**the primary use of the EIGRP RID is to prevent routing loops. The RID is used to identify the originating router for external routes。假如接收到一条有着与本地路由器一致的RID外部路由那么就会将其丢弃。设计此特性的目的就是降低那些其中有着多台**自治系统边界路由器**AS Boundary Router, ASBR正进行路由重分发的网络中出现路由环回的可能性。
在确定RID时**EIGRP将选取路由器上所配置的IP地址中最高的作为RID**。**但如果在路由器上配置了环回接口,那么将优先选取这些接口,因为环回接口是路由器上存在的最稳定接口**。除非将EIGRP进程移除那么RID随后就绝不会变化了也就是假如RID是手动配置的情况。RID始终会在EIGRP拓扑表中列出如下所示
```console
R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(150)/ID(10.3.3.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.2.2.0/24, 1 successors, FD is 128256
via Connected, Loopback2
P 10.3.3.0/24, 1 successors, FD is 128256
via Connected, Loopback3
P 10.1.1.0/24, 1 successors, FD is 128256
via Connected, Loopback1
P 10.0.0.0/24, 1 successors, FD is 128256
via Connected, Loopback0
P 150.1.1.0/24, 1 successors, FD is 2169856
via Connected, Serial0/0
```
> **注意:** 这里重要的是掌握到RID与邻居ID通常是不同的然而这对于那些比如只有一个接口的路由器可能不适用。It is important to understand that the RID and the neighbour ID will typically be different, although this may not be the case in routers with a single interface, for example
EIGRP的路由器IDRID是通过路由器配置命令`eigrp router-id [address]`进行配置的。在输入了此命令后RID就以这个新地址在EIGRP的拓扑表中得以更新。为对此进行演示这里就以查看路由器上的当前RID开始如下面的拓扑表中所指出的
```console
R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(150)/ID(10.3.3.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
...
[Truncated Output]
```
现在路由器上配置一个`1.1.1.1`的RID如下所示
```console
R1(config)#router eigrp 150
R1(config-router)#eigrp router-id 1.1.1.1
R1(config-router)#
*Mar 1 05:50:13.642: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 150: Neighbor 150.1.1.2 (Serial0/0) is down: route configuration changed
*Mar 1 05:50:16.014: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 150: Neighbor 150.1.1.2 (Serial0/0) is up: new adjacency
```
伴随这个改变EIGRP邻居关系就被重置了同时在EIGRP拓扑表中立即反映出了这个新的RID如下所示
```console
R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(150)/ID(1.1.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
...
[Truncated Output]
```
在对EIGRP的路由器IDRID进行配置时应记住下面两点
- 不能无法将RID配置为`0.0.0.0`
- 不能无法将RID配置为`255.255.255.255`
现在源自该路由器的所有外部路由就都包含了这个EIGRP路由器ID了。在下面的邻居路由器`R2`输出中,就可对此进行验证:
```console
R2#show ip eigrp topology 192.168.254.0/24
IP-EIGRP (AS 150): Topology entry for 192.168.254.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 7289856
Routing Descriptor Blocks:
150.1.1.1 (Serial0/0), from 150.1.1.1, Send flag is 0x0
Composite metric is (7289856/6777856), Route is External
Vector metric:
Minimum bandwidth is 1544 Kbit
Total delay is 220000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
External data:
Originating router is 1.1.1.1
AS number of route is 0
External protocol is Connected, external metric is 0
Administrator tag is 0 (0x00000000)
```
而对于内部EIGRP路由则是不包含RID的如下面的输出所示
```console
R2#show ip eigrp topology 10.3.3.0/24
IP-EIGRP (AS 150): Topology entry for 10.3.3.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2297856
Routing Descriptor Blocks:
150.1.1.1 (Serial0/0), from 150.1.1.1, Send flag is 0x0
Composite metric is (2297856/128256), Route is Internal
Vector metric:
Minimum bandwidth is 1544 Kbit
Total delay is 25000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
```
## 第36天问题
1. You can see the ASN with the `show ip _______` command.
2. Every router you want to communicate with in your routing domain must have a different ASN. True or false?
3. What is the purpose of the EIGRP topology table?
4. By default, EIGRP uses the `_______` bandwidth on the path to a destination network and the total `_______` to compute routing metrics.
5. Dynamic neighbour discovery is performed by sending EIGRP Hello packets to the destination Multicast group address `_______`.
6. EIGRP packets are sent directly over IP using protocol number `_______`.
7. To populate the topology table, EIGRP runs the `_______` algorithm.
8. The `_______` `_______` includes both the metric of a network as advertised by the connected neighbour, plus the cost of reaching that particular neighbour.
9. Cisco IOS software supports equal cost load sharing for a default of up to four paths for all routing protocols. True or false?
10. What EIGRP command can be used to enable unequal cost load sharing?
## 第36天问题答案
1. `protocols`.
2. False.
3. The topology table allows all EIGRP routers to have a consistent view of the entire network. All known destination networks and subnets that are advertised by neighbouring EIGRP routers are stored there.
4. Minimum, delay.
5. `224.0.0.10`.
6. 88.
7. DUAL.
8. Feasible Distance.
9. True.
10. The `variance` command.
## 第36天实验
**EIGRP的实验**
### 拓扑图
![EIGRP实验拓扑图](images/3622.png)
### 实验目的
学习如何配置基本的EIGRP。
### 实验步骤
1. 基于上面的拓扑配置上所有IP地址。确保可以经由串行链路`ping`通。
2. 在两台路由器上以自治系统编号30, 配置EIGRP。
```console
RouterA(config)#router eigrp 30
RouterA(config-router)#net 172.20.0.0
RouterA(config-router)#net 10.0.0.0
RouterA(config-router)#^Z
RouterA#
RouterB#conf t
Enter configuration commands, one per line.
End with CNTL/Z.
RouterB(config)#router eigrp 30
RouterB(config-router)#net 10.0.0.0
%DUAL-5-NBRCHANGE: IP-EIGRP 30: Neighbor 10.0.0.1 (Serial0/1/0) is up: new adjacency
RouterB(config-router)#net 192.168.1.0
```
3. 对两台路由器上的路由表分别进行检查。
```console
RouterA#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 10.0.0.0/8 is a summary, 00:01:43, Null0
C 10.0.0.0/30 is directly connected, Serial0/1/0
172.20.0.0/16 is variably subnetted, 2 subnets, 2 masks
D 172.20.0.0/16 is a summary, 00:01:43, Null0
C 172.20.1.0/24 is directly connected, Loopback0
D 192.168.1.0/24 [90/20640000] via 10.0.0.2, 00:00:49, Serial0/1/0
RouterA#
```
```console
RouterB#show ip route
...
[Truncated Output]
...
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 10.0.0.0/8 is a summary, 00:01:21, Null0
C 10.0.0.0/30 is directly connected, Serial0/1/0
D 172.20.0.0/16 [90/20640000] via 10.0.0.1, 00:01:27, Serial0/1/0
192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
D 192.168.1.0/24 is a summary, 00:01:21, Null0
C 192.168.1.0/26 is directly connected, Loopback0
RouterB#
```
4. 查明两台路由器都对各个网络进行着自动汇总。并于随后在路由器B上关闭自动汇总。
```console
RouterB#show ip protocols
Routing Protocol is “eigrp 30”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 30
Automatic network summarization is in effect
Automatic address summarization:
192.168.1.0/24 for Serial0/1/0
Summarizing with metric 128256
10.0.0.0/8 for Loopback0
Summarizing with metric 20512000
Maximum path: 4
Routing for Networks:
10.0.0.0
192.168.1.0
Routing Information Sources:
Gateway Distance Last Update
10.0.0.1 90 496078
Distance: internal 90 external 170
RouterB(config)#router eigrp 30
RouterB(config-router)#no auto-summary
```
5. 对路由器A上的路由表进行检查。
```console
RouterA#show ip route
...
[Truncated Output]
...
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 10.0.0.0/8 is a summary, 00:00:04, Null0
C 10.0.0.0/30 is directly connected, Serial0/1/0
172.20.0.0/16 is variably subnetted, 2 subnets, 2 masks
D 172.20.0.0/16 is a summary, 00:00:04, Null0
C 172.20.1.0/24 is directly connected, Loopback0
192.168.1.0/26 is subnetted, 1 subnets
D 192.168.1.0 [90/20640000] via 10.0.0.2, 00:00:04, Serial0/1/0
RouterA#
```
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