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）。
- 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ID，Router 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, RIP），IGRP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。
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）了，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EIGRP，EIGRP 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：
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
`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。
`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）。
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TLV，the Sequence TLV, which is used by RTP
- 下一次多播序号TLV，RTP使用的TLV，the 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
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：
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)
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：
*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。
- 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
> **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。
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。
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）:
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）时，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。
`%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。
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） |
| 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：
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：
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。
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**）。
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。
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
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。
> **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）。
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：
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。
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
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**）。
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.
> **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**）。
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（multiplier）1，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。
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）。
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
`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：
*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）。
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
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：
*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）中，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,
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。
- 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：
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：
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：
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）：
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
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
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：
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
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：
*图 36.18 -- 不良路由汇总下的流量黑洞问题，Black-Holing traffic with Poor Route Summarisation*
*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）。
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,
与OSPF使用路由器ID（the 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。
> **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）。
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
### 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
