EIGRP协议详解与故障排查

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"Enhanced Interior Gateway Routing Protocol.pdf 是一篇关于Cisco设备中增强型内部网关路由协议（EIGRP）的详细技术文档。该文档涵盖了EIGRP的基本理论、操作机制、主要版本更新、邻居发现与维护、拓扑表构建、度量计算、路径选择策略、故障排查以及与其他路由协议的重分布和汇总策略等多个方面。\n\nEIGRP是一种先进的距离矢量路由协议，设计用于解决早期IGRP（内部网关路由协议）的一些局限性。其核心理论在于动态适应网络变化，提供高效、快速的路由收敛和无环路路径选择。\n\n在协议的主要修订部分，文档可能涉及EIGRP自适应度量模型的改进，以及为了提高性能和稳定性所做的各种调整。\n\n在基本理论部分，文档详细解释了EIGRP如何通过邻居发现和维护来建立邻接关系，以及如何使用这些信息来构建和更新拓扑表。拓扑表是EIGRP选择最佳路径的基础。\n\nEIGRP的度量包括带宽、延迟、可靠性、负载和最大传输单元（MTU），这些因素共同决定了可行距离（Feasible Distance, FD）和报告距离（Reported Distance, RD）。可行后继（Feasible Successor）算法用于确保选择的路径是无环路的。同时，EIGRP采用诸如分段水平法（Split Horizon）和毒反向（Poison Reverse）等策略防止路由环路的形成。\n\n在启动模式和拓扑表变更中，文档可能会介绍EIGRP如何处理网络变动，如何查询丢失或不可达的路由，以及遇到“卡在活动”状态（Stuck In Active, SIA）的路由时的处理方法。\n\n重分布是EIGRP的关键特性之一，文档将详述如何在两个EIGRP自治系统之间、EIGRP和IGRP在不同或相同自治系统间，以及EIGRP和其他协议之间进行路由信息的重分布。这涉及到确保路由信息的正确导入和导出，以保持网络的稳定运行。\n\n汇总（Summarization）是EIGRP优化路由信息传播的手段，包括自动汇总和手动汇总。自动汇总通常应用于外部路由，而范围查询处理和汇总点的选择会直接影响查询范围的大小。此外，文档还可能讨论了分布列表对查询范围的影响。\n\n最后，文档可能涵盖EIGRP的包调度（Pacing Packets）和默认路由设置，这些都是保证网络效率和可靠性的关键要素。通过理解并熟练应用这些概念，网络管理员可以更有效地管理和优化使用EIGRP的网络环境。"

There are two routes to Network A from Router One: one through Router Two with a metric of 46789376 and

another through Router Four with a metric of 20307200. Router One chooses the lower of these two metrics

as its route to Network A, and this metric becomes the feasible distance. Next, let us look at the path through

Router Two to see if it qualifies as a feasible successor. The reported distance from Router Two is 46277376,

which is higher than the feasible distance − so this path is not a feasible successor. If you were to look in the

topology table of Router One at this point (using show ip eigrp topology), you would only see one entry for

Network A − through Router Four. (In reality there are two entries in the topology table at Router One, but

only one will be a feasible successor, so the other will not be displayed in show ip eigrp topology; you can

see the routes that are not feasible successors using show ip eigrp topology all−links).

Let us suppose that the link between Router One and Router Four goes down. Router One sees that it has lost

its only route to Network A, and queries each of its neighbors (in this case, only Router Two) to see if they

have a route to Network A. Since Router Two does have a route to Network A, it responds to the query. Since

Router One no longer has the better route through Router Four, it accepts this route through Router Two to

Network A.

Deciding if a Path is Loop−Free

How does EIGRP use the concepts of feasible distance, reported distance, and feasible successor to determine

if a path is valid, and not a loop? In Figure 4a, Router Three examines routes to Network A. Since split

horizon is disabled (for example, if these are multipoint Frame Relay interfaces), Router Three shows three

routes to Network A: through Router Four, through Router Two (path is two, one, three, four), and through

Router One (path is one, two, three, four).

If Router Three accepts all of these routes, it results in a routing loop. Router Three thinks it can get to

Network A through Router Two, but the path through Router Two passes through Router Three to get to

Network A. If the connection between Router Four and Router Three goes down, Router Three believes it can

get to Network A through one of the other paths, but because of the rules for determining feasible successors,

it will never use these paths as alternates. Let us look at the metrics to see why:

total metric to Network A through Router Four: 20281600•

total metric to Network A through Router Two: 47019776•

total metric to Network A through Router One: 47019776•

Since the path through Router Four has the best metric, Router Three installs this route in the forwarding table

and uses 20281600 as its feasible distance to Network A. Router Three then computes the reported distance to

Network A through Routers Two and One: 47019776 for the path through Router Two, and 47019776 for the

path through Router One. Because both of these metrics are greater than the feasible distance, Router Three

does not install either route as a feasible successor for Network A.

Suppose that the link between Routers Three and Four goes down. Router Three queries each of its neighbors

for an alternative route to Network A. Router Two receives the query and, because the query is from its

successor, searches each of the other entries in its topology table to see if there is a feasible successor. The

only other entry in the topology table is from Router One, with a reported distance equal to the last known

best metric through Router Three. Because the reported distance through Router One is not less than the last

known feasible distance, Router Two marks the route as unreachable and queries each of its neighbors − in

this case, only Router One − for a path to Network A.

Router Three also sends a query for Network A to Router One. Router One examines its topology table and

finds that the only other path to Network A is through Router Two with a reported distance equal to the last

known feasible distance through Router Three. Once again, since the reported distance through Router Two is

not less than the last known feasible distance, this route is not a feasible successor. Router One marks the

route as unreachable and queries its only other neighbor, Router Two, for a path to Network A.

This is the first level of queries. Router Three has queried each of its neighbors in an attempt to find a route to

Network A. In turn, Routers One and Two have marked the route unreachable, and queried each of their

remaining neighbors in an attempt to find a path to Network A. When Router Two receives the Router One

query, it examines its topology table and notes that the destination is marked as unreachable. Router Two

replies to Router One that Network A is unreachable. When Router One receives the Router Two query, it

also sends back a reply that Network A is unreachable. Now Routers One and Two have both concluded that

Network A is unreachable, and they reply to the original Router Three query. The network has converged, and

all routes return to the passive state.

Split Horizon and Poison Reverse

In the previous example, we assumed that split horizon was not in effect to show how EIGRP uses the feasible

distance and the reported distance to determine if a route is likely to be a loop. In some circumstances,

however, EIGRP uses split horizon to prevent routing loops as well. Before dealing with the details of how

EIGRP uses split horizon, let us review what split horizon is and how it works. The split horizon rule states:

Never advertise a route out of the interface through which you learned it.•

For instance, in Figure 4a, if Router One is connected to Routers Two and Three through a single multipoint

interface (such as Frame Relay), and Router One learned about Network A from Router Two, it will not

advertise the route to Network A back out the same interface to Router Three. Router One assumes that

Router Three would learn about Network A directly from Router Two.

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njpostboy

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