EIGRP Convergence

EIGRP convergence occurs rapidly in an internetwork, due in no small part to EIGRP supporting up to six parallel paths to all destinations in the network.

EIGRP convergence time is very low, based on both the RTP (Reliable Transport Protocol) and EIGRP update packets. Rapid convergence is based upon the feasible successors, which are determined by the DUAL engine.

The DUAL finite-state machine uses distance information to select efficient, loop-free paths, and it selects routes for insertion in a routing table based on feasible successors. The feasible successor is a neighboring router used for packet forwarding that is a least-cost path to a destination guaranteed not to be part of a routing loop. When a neighbor changes a metric, or when a topology change occurs, DUAL tests for feasible successors. If one is found, DUAL uses it to avoid recomputing the route unnecessarily. When no feasible successors exist but neighbors still advertise the destination, a recomputation (also known as a diffusing computation) must occur to determine a new successor. Although recomputation is not processor-intensive, it does affect convergence time, so it is best to avoid unnecessary recomputations.

Because of the combination of the DUAL finite-state machine and the EIGRP topology table packets, EIGRP convergence can occur within a few seconds, making it the premier choice for Cisco internetworks.



Network Sales and Services Handbook
Network Sales and Services Handbook (Cisco Press Networking Technology)
ISBN: 1587050900
EAN: 2147483647
Year: 2005
Pages: 269

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