Figure 111 A small EIGRP network

Hetwrif 133.9.3

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In this illustration, there are five different networks within a single EIGRP Autonomous System. The routers are labeled A through E. Another simplifying assumption in this example is that all the links are T1s. Therefore, bandwidth and delay will be constant throughout the network, and route selection becomes an obfuscated hop-counting exercise. Therefore instead of complicating the example with composite metrics, hop counts are used.

Table 11-1 contains the known distances between Router C and the other networks.

Table 11-1: Distances from Router C in the Network

Destination IP Address

Next Hop

Hop Count

193.9.1

A

1

193.9.2

B

1

193.9.4

B

2

193.9.5

A

2

Router C's DUAL algorithm has selected the least-cost paths from the multiple available paths to networks 193.9.4 and 193.9.5. Table 11-2 summarizes Router C's view of the network. Note that none of these routers have any feasible successors for any of these destinations. This is because the distance reported by the neighbor must be less than---not less than or equal to---the best metric available to reach that destination.

Table 11-2: A Summary of Router C's Network Topology

Destination IP Address

Route, from C

Hop Count

Successor or Feasible Successor?

193.9.1

A

1

Successor

193.9.2

B

1

Successor

193.9.4

B to D

2

Successor

193.9.5

A to E

2

Successor

There are other paths through the network, but their hop counts exceed both the primary route and the feasible successor. It is possible, for example, for Router C to forward datagrams to network 193.9.1

Network 5

by using the route through Router B to D to E and, finally, to Router A and network 193.9.1. However, the hop count of this network (where both bandwidth and delay are equal across all links) is four. Therefore, it is unattractive as both a primary route and a feasible successor to the primary route. Such a route may become either a primary route or a feasible successor, but only if multiple network failures occurred and it were the least-cost route. However, the entire EIGRP network would have to recompute routes to known destinations for this to occur. To understand how the process of finding an alternative path works, consider Figure 11-2. In this illustration, the link between Routers B and C fails.

Figure 11-2: The link between Routers C and B fails.

Figure 11-2: The link between Routers C and B fails.

The consequences of this failure are that all routes that used B as a next hop go active in the EIGRP topology table. The effects of this failure, as documented in the topology table, are summarized in Table 11-3.

Table 11-3: A Summary of Router C's Network Topology, after a Link Failure

Destination IP Address

Route, from C

Route State

Successor or Feasible Successor?

193.9.1

A

Passive

Successor

193.9.2

B

Active

Successor

193.9.4

B to D

Active

Successor

193.9.5

A to E

Passive

Successor

In this example, all that used the link between Routers C and B become active in the topology table. Other routes, including those that pass through Router B via Router A, remain passive and unaffected by the topology change.

Router C responds to this topology change by sending a query to its neighbors, notifying them that it has lost two primaries. It has only two neighbors, B and A, and one of them is now unreachable.

Router A is obligated by the protocol specifications to respond to Router C's query for alternative path information. Its own topology table has not been affected by the link failure because it has a different set of neighbors. Therefore, there is hope that other routes can be discovered.

Enhanced IGRP

Router A's topology table, in the middle of convergence, is summarized in Table 11-4. Table 11-4: A Summary of Router A's Network Topology, in the Midst of Convergence

Destination IP Address

Route, from A

Status

Successor or Feasible Successor?

193.9.2

B

Passive

Successor

193.9.3

C

Passive

Successor

193.9.4

B to D

Passive

Successor

193.9.5

E

Passive

Successor

The link failure between Routers B and C has not affected any of Router A's primary routes. They remain passive and in use. Because this is the case, Router A will respond with information on an alternative route through Router E to these destination networks.

When Router C receives the reply from Router A, it knows that all the neighbors in the network have processed the link failure and modified their tables accordingly.

Table 11-5 summarizes the results of Router C's new understanding of the network's topology.

Table 11-5: A Summary of Router C's Network Topology, Post Convergence

Destination IP Address

Route, from C

State

Successor or Feasible Successor?

193.9.1

A

Passive

Successor

193.9.2

A to B

Passive

Successor

193.9.4

A to B to D

Passive

Successor

193.9.5

A to E

Passive

Successor

Router C was able to identify an alternative path---that is, successor---to all the routes it had been able to reach through Router B. These alternatives are far from ideal, however: They all begin with the hop to Router A, which is also the primary route to 193.9.1. If a failure were to beset this link, Router A, or any of the router interfaces that connect this link to Routers A and C, Router C would be completely isolated from the remainder of the network.

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