The router will not reply to the querying router until it has received a reply to all queries that it originated for this route. If the EIGRP topology table lists the querying router as the successor for this route and a feasible successor does not exist, then the router queries all of its EIGRP neighbors except those sent out the same interface as its former successor. If the EIGRP topology table lists the querying router as the successor for this route and a feasible successor exists, then the feasible successor is installed and the router immediately replies to the query. If the EIGRP topology table does not currently contain an entry for the route, then the router immediately replies to the query with an unreachable message, stating that there is no path for this route through this neighbor. When an EIGRP neighbor receives a query for a route, it behaves as follows: In the active state, a router sends queries out to its neighbors requesting a path to the lost route. If, for some reason, a router loses a route through its successor and does not have a feasible successor for that route, then the route transitions to an “active” state. Routes that have a valid successor are said to be in a “passive” state. Conventionsįor more information on document conventions, refer to the Cisco Technical Tips Conventions. If your network is live, make sure that you understand the potential impact of any command. All of the devices used in this document started with a cleared (default) configuration. The information in this document was created from the devices in a specific lab environment. The information in this document is based on Cisco IOS® Software Release 12.0. There are no specific requirements for this document. To reduce traffic load on the network, EIGRP maintains neighbor relationships and exchanges routing information only as needed, using a query process to find alternate paths when all loop-free paths to a destination have failed. The remaining neighbors with loop-free paths to the destination are called feasible successors. The neighbor (or neighbors) with the best path to a destination is called the successor. It is capable of (conservatively) finding all loop-free paths to any given destination based on route advertisements from neighbors. Enhanced Interior Gateway Routing Protocol (EIGRP) is an enhanced distance-vector protocol based on the diffusing update algorithm (DUAL).
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