What are a successor and a feasible successor in EIGRP?
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The successor is the neighbor on the best loop-free path to a prefix; that path goes into the routing table. A feasible successor is a backup neighbor that DUAL has proven loop-free, because its reported distance is lower than my feasible distance. When the successor fails, the feasible successor is promoted immediately, with no queries and no waiting.
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Where to see them
R1 has three neighbors toward 10.50.0.0/24: R2 on Gi0/0/0, R3 on Gi0/0/1 and R4 on Gi0/0/2.
R1# show ip eigrp topology
EIGRP-IPv4 Topology Table for AS(100)/ID(10.255.0.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.50.0.0/24, 1 successors, FD is 3328
via 10.0.12.2 (3328/3072), GigabitEthernet0/0/0
via 10.0.13.3 (26368/3072), GigabitEthernet0/0/1- The first
vialine is the successor (R2). Its total metric, 3328, is the FD. - The second line is a feasible successor (R3): its reported distance 3072 is less than 3328.
- R4 is missing. Its path fails the feasibility condition, and the plain command only lists
successors and feasible successors.
show ip eigrp topology all-linksshows it.
Why it matters
A feasible successor gives sub-second failover with no network-wide conversation. No feasible successor means the route goes active and the router queries its neighbors, which is slower and is the root of stuck-in-active problems.
They will ask next
Can there be more than one successor?
Yes. Paths with exactly the same best metric are all successors and all go into the routing table, up to
maximum-paths, which defaults to 4.Is the feasible successor in the routing table?
Not by default. It sits in the topology table ready to be promoted. It only goes into the routing table if you configure
variancehigh enough to include it.
Traps
- Calling the feasible successor "the second best path". It is a path that passed the feasibility condition, which is not always the next best by metric.