EIGRP Successor and Feasible Successor Explained

Learn how EIGRP selects successor routes, evaluates feasible successors with RD < FD, and converges after a primary path fails.

EIGRP (Enhanced Interior Gateway Routing Protocol) can learn multiple paths to the same destination prefix from different neighbors. It evaluates those paths, installs the best one for forwarding, and may retain a prequalified backup for rapid recovery.

Two EIGRP route roles are central to this process: the successor and the feasible successor. The successor is the active best route. A feasible successor is an alternate route that has passed EIGRP's loop-free test.

EIGRP Route-Selection Context

A router can receive information about the same destination from several EIGRP neighbors. For example, R1 might learn paths to 10.0.0.0/24 through R2, R3, and R4.

EIGRP keeps learned route information in its topology table. The route selected for normal forwarding is placed in the IP routing table. These are different data structures:

• The topology table contains learned paths, metrics, successor information, and qualifying backup paths.
• The routing table contains active routes used to forward packets.

The path with the lowest local EIGRP metric is selected as the best path. Other paths are not automatically usable backups. Each alternate must satisfy EIGRP's feasibility condition before it can become a feasible successor.

What Is a Successor?

A successor is the EIGRP route with the lowest total metric from the local router to a destination. It is the primary next-hop path selected for forwarding.

The successor route is installed in the IP routing table. In a basic single-path example, one neighbor provides the successor for a destination. If multiple paths have equal cost, EIGRP can install more than one successor and perform equal-cost load balancing.

The local router's total metric through a neighbor is called the feasible distance (FD). For the selected successor, this value represents the best known total EIGRP metric from the local router to the destination.

What Is a Feasible Successor?

A feasible successor is a prequalified alternate route to the same destination. EIGRP has determined that the route is loop-free and can be used if the current successor becomes unavailable.

Under normal conditions, a feasible successor is retained in the EIGRP topology table rather than installed as the active route in the IP routing table. If the successor fails, EIGRP can promote the feasible successor immediately because it has already passed the loop-free test.

Not every alternate route is a feasible successor. A route can appear in EIGRP topology information and still fail to qualify.

Feasible Distance and Reported Distance

Feasible distance (FD)

Feasible distance is the local router's calculated total EIGRP metric to a destination through a particular route. It includes the cost from the local router toward its neighbor and the neighbor's distance onward to the destination.

Reported distance (RD)

Reported distance, also called advertised distance (AD), is the metric a neighboring router reports for its own path to the destination. It describes the neighbor's view of its distance to that destination; it is not the complete local metric from the receiving router.

Do not treat RD and FD as interchangeable:

• FD: the local router's total metric to the destination through a route.
• RD: the neighbor's reported metric to the destination.

When EIGRP tests an alternate path, it compares the candidate neighbor's RD with the current successor's FD.

The Feasibility Condition

The feasibility condition (FC) is:

RD < FD

For a candidate path to qualify as a feasible successor, the neighbor's reported distance must be strictly less than the local router's feasible distance for the current successor.

The less-than operator is important. An RD equal to the successor FD does not qualify:

RD = FD  ->  not a feasible successor

The condition provides a loop-free guarantee. If a neighbor reports a distance lower than the local router's known distance through the successor, that neighbor has a path that does not depend on routing back through the local router for this destination. Therefore, EIGRP can use that neighbor as a prequalified alternate without first performing a new route search.

Worked Example: Classifying Three Paths at R1

Assume R1 needs to reach destination network 10.0.0.0/24, which is originated or advertised by R5. R1 learns possible paths through neighbors R2, R3, and R4.

R1 selects the path through R2 as the successor. Its feasible distance is 30. R1 then evaluates alternate neighbors by comparing each neighbor's reported distance with 30.

Interpreting the results

1. The R2 path has the lowest total metric selected by R1, so it becomes the successor. R1 installs it in the routing table.
2. The R3 path is an alternate known to EIGRP, but R3 reports a distance of 50. Since 50 < 30 is false, it fails the feasibility condition.
3. The R4 path has a reported distance of 25. Since 25 < 30 is true, it qualifies as a feasible successor and remains available in the topology table.

Notice that an alternate path can have a higher total metric than the successor and still be a feasible successor. Feasible-successor status depends on the candidate neighbor's RD compared with the successor's FD, not simply on whether the candidate is second-best by total metric.

Boundary Condition: Equal RD and FD

Suppose a candidate neighbor reports a distance of 30 while the successor's feasible distance is also 30:

Candidate RD = 30
Successor FD = 30
30 < 30 = false

The candidate does not qualify. EIGRP requires a strict inequality, RD < FD, not RD <= FD.

Failure and Convergence Behavior

If the R2 successor in the example fails and the R4 feasible successor is still available, EIGRP can promote the R4 path quickly. R4 was already verified as a loop-free alternate, so R1 does not need to first discover and validate a replacement route.

If the successor fails and no feasible successor exists, EIGRP must perform additional route computation and query behavior to find another valid path. This takes longer than promoting an already qualified feasible successor.

Prequalified loop-free backups improve convergence because the router can recover without waiting for a new route search to complete.

Successor vs. Feasible Successor

Verifying Successors and Feasible Successors

View active EIGRP routes

show ip route eigrp

Use this command to identify EIGRP routes installed in the IP routing table. An installed EIGRP route represents an active successor path, or one of multiple equal-cost successors.

Inspect the EIGRP topology table

show ip eigrp topology

This output shows learned destination information and route metrics. Use it to compare the successor's feasible distance with the reported distances of alternate neighbors and to identify feasible-successor candidates.

Inspect one destination in detail

show ip eigrp topology 10.0.0.0 255.255.255.0

This focused command helps evaluate the paths for 10.0.0.0/24. Locate the successor's FD, then compare each candidate neighbor's RD against it using RD < FD.

Common Mistakes and Troubleshooting

An alternate path is visible but is not an immediate backup

The path may be present in the topology table but fail the feasibility condition. Compare its RD with the current successor FD. If the RD is greater than or equal to the FD, it is not a feasible successor.

Route replacement is not immediate after a failure

No feasible successor may have been available when the successor failed. Review the destination's topology entry and determine whether any alternate path had passed RD < FD. Without a prequalified backup, EIGRP must seek and calculate another valid route.

The lowest reported distance was incorrectly labeled the successor

A neighbor's RD is not the local router's complete path metric. First compare the local total metrics to select the successor. Then use each candidate's RD only for the feasibility-condition test.

Key Takeaways

• The successor is the currently selected best EIGRP route and is installed in the routing table.
• A feasible successor is a loop-free alternate retained in the topology table.
• Feasible distance is the local router's total metric to the destination.
• Reported distance, also called advertised distance, is the neighbor's metric to the destination.
• The feasibility condition is strictly RD < FD.
• An alternate route may have a higher total metric and still qualify as a feasible successor.
• If a feasible successor exists when the successor fails, EIGRP can converge quickly by promoting it.
• If no feasible successor exists, EIGRP must perform additional route computation and query behavior.

For related details, review successor and feasible successor concepts together with EIGRP topology-table verification.