6.5 Enhanced IGRP Operation
6.5.4 Maintaining routes
When there is a change in the network, the router that learned about the change advertises it to its neighbors by multicasting an update packet with the change. If the update packets are to notify the neighbors that a router was added to the network, then the process described in the previous "Discovering Neighbors" and "Discovering Routes" sections occurs. However, if the update packet says that a link has a worse metric, or is no longer available, the router must find an alternative path.

To obtain an alternative path, the router that lost the link looks for a new feasible successor in its topology table. If a feasible successor exists, it is promoted to a successor and added to the routing table, and then used. The topology table is then recalculated to determine whether there are any new feasible successors, based on the new successor's feasible distance.

If a feasible successor is not available, the following process is performed. -

  1. The router (router A) flags the failed route as in an "active" state in the topology table. When routes are operating well, they are in "passive" state.
  2. Router A looks for an alternative path by sending out a query packet to all its neighbors to learn whether they have a path to the given destination. The query packet is multicast out every interface except the one from which the dead link was learned, thus following the split horizon rule.

Because the router expects a reply to the query from each neighbor, it tracks the sending and receiving of these packets from topology table.

In Figure , for example, no feasible successor exists because no router's advertised distance is less than router B's feasible distance. As a result, the router must query its neighbors to find new successors and feasible successors. The route to network 7 changes from passive to active state.

  1. If a neighbor has a feasible successor that does not use the querying router, or no route at all to the destination, it unicasts a Reply packet to the requestor indicating the appropriate information.

If a neighbor that receives the query is using the querying router as its feasible successor, then it sends its own Query packet to its neighbors, which creates a query ripple effect through the network until a major network boundary is met with, or until the router is on the autonomous system boundary (the end of EIGRP routers). In Figure , for example, you see router B send a query to its next network.

  1. When the query router receives replies, it reacts, based on the answer in the reply:
  • If the reply included a successor or feasible successor, the information is put into its topology table and the querying router waits until all replies are received. It then recalculates the topology table and adds the successor(s) to the routing table. The route returns to a passive state in the topology table and routing can continue.
  • If none of the replies includes a successor or feasible successor, the querying router removes the active route from its topology and routing tables.

If one or more routers to which a query is sent do not respond with a reply within the active time of 180 seconds, EIGRP tears down the neighbor relationship with this rogue router and puts routes that used the rogue router into an active state. The querying router then generates queries for the route(s) it lost through the rogue router.

Lab Activity
  In this lab, you will learn how to use VLSM and IP unnumbered with the EIGRP routing protocol.