EIGRP Loop Avoidance

EIGRP avoids loops by keeping some basic topological information but not full information. When a router learns multiple routes to the same subnet, it puts the best route in the routing table, following the same rules about adding multiple equal-metric routes as IGRP. In EIGRP, the best route, i.e., the route with the lowest metric is called the successor. EIGRP also runs an algorithm to identify which backup routes could be used in case of a route failure, without causing a loop. These routes are called feasible successors.

Should the best route (successor) fail and there are no feasible successors for that route, EIGRP uses a distributed algorithm called Diffusing Update Algorithm (DUAL). DUAL sends queries looking for a loop-free route to the subnet in question. When the new route is found, DUAL adds it to the routing table.