Neighbors, metrics and topology updates
EIGRP uses Hellos to discover and maintain neighbors. A new neighbor receives an initial exchange of routing information. After that, Update messages carry changes as needed. EIGRP uses IP protocol 88 and can carry IPv4 or IPv6 routes.
By default the composite metric uses the lowest bandwidth and cumulative configured delay along a path. The topology table tracks alternatives learned from neighbors. DUAL, the Diffusing Update Algorithm, decides which can serve as loop-free next hops.
A feasible successor can take over immediately
A successor is a next hop chosen by DUAL to forward traffic to the destination. A feasible successor is a candidate whose reported distance, the neighbor’s own metric to the destination, is strictly below the local feasible distance. Feasible distance is the lowest local metric recorded since the route last transitioned from Active to Passive, and can differ from the current best metric.
Suppose the local feasible distance is 100. A backup neighbor reporting 80 can qualify even when the local metric through it is 130. A neighbor reporting 100 fails the strict comparison. If the successor fails, EIGRP can switch to a feasible successor without querying for that destination. If no feasible successor is available, DUAL can mark the route Active and query neighbors before settling on a new path.
Try it in a lab
- On EIGRP-capable lab routers, give one router two paths to the same prefix. Adjust the link metrics until the topology table shows a successor and a feasible successor.
- Record each neighbor’s reported distance and the feasible distance. Disable the primary path and confirm the route switches to the feasible successor.
- Repeat with an alternate whose reported distance fails the feasibility condition. Observe the query and reply exchange after the primary path fails, then check the selected route after convergence.