3.5.h Implement, troubleshoot and optimize EIGRP convergence and scalability

3.5.h [ii] Control query boundaries

EIGRP is an advanced distance-vector protocol; it doesn’t have LSA flooding like OSPF. EIGRP relies only on its neighbors for information on network reachability and availability. EIGRP keeps a list of backup routes called feasible successors. When the primary route is not available, EIGRP immediately uses the feasible successor as the backup route. This shortens convergence time. Now, if the primary route is gone and no feasible successor is available, the route is in active state. The only way for EIGRP to converge quickly is to query its neighbors about the unavailable route. If the neighbor doesn’t know the status of the route, the neighbor asks its neighbors, and so on, until the edge of the network is reached.

The query stops if one of the following occurs:

● All queries are answered from all the neighbors.

● The end of network is reached.

● The lost route is unknown to the neighbors.

The problem is that, if there are no query boundaries, EIGRP potentially can ask every router in the network for a lost route. When EIGRP first queries its neighbor, a stuck in active timer starts. By default, the timer is 180 seconds. If, in three minutes, EIGRP doesn’t receive the query response from all its neighbors, EIGRP declares that the route is stuck in active state and resets the neighbor that has not responded to the query.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3259-3271).  . Kindle Edition.

http://blog.ine.com/2009/08/02/eigrp-query-scoping/