6.4 Convergence Using EIGRP
6.4.2 Alternative paths
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 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 . In this illustration, the link between routers B and C 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 Figure .

In this example, all routes 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 the router C 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.

The router A topology table, in the middle of convergence, is summarized in Figure .

The link failure between routers B and C has not affected any of the router A primary routes. They remain passive and in use. Therefore, 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.

Figure summarizes the results of the new understanding that router C has of the network topology.

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 occur on this link, router A, or on any of the router interfaces that connect this link to routers A and C, router C would be completely isolated from the rest of the network.