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EIGRP Successor and Feasible Successor Explained

Learn how EIGRP selects a successor, evaluates feasible successors with FD and RD, prevents loops, and reacts when a primary route fails.

EIGRP can learn several paths to the same destination from different neighbors. It does not automatically install every learned path in the IP routing table. Instead, EIGRP uses the Diffusing Update Algorithm (DUAL) and its metric rules to select a best path and identify any loop-free backup paths.

The two key terms are successor and feasible successor. The successor is the preferred path used for forwarding. A feasible successor is a prequalified backup that can be used immediately if the successor fails.

EIGRP route selection overview

For each destination prefix, such as 10.0.0.0/24, an EIGRP router may receive route information from multiple neighbors. EIGRP evaluates that prefix independently from other prefixes.

EIGRP keeps learned information in two important places:

• Topology table: The EIGRP database containing destination information, neighbor paths, metrics, successor data, and possible backup paths.
• IP routing table: The forwarding table used to decide where packets are sent. Only selected routes are normally installed here.

The topology table can therefore contain more paths than the routing table. An alternate path may be known to EIGRP without being an active forwarding route.

In a basic comparison, EIGRP first looks for the path with the lowest calculated composite metric. That path becomes the successor if it is a valid loop-free route. EIGRP then applies the feasibility condition to alternate paths to determine whether they qualify as feasible successors.

What is an EIGRP successor?

A successor is the best loop-free EIGRP path to a destination. In the normal case, it is the path with the lowest feasible distance (FD).

The successor is installed in the IP routing table and used for packet forwarding. EIGRP may install more than one successor when paths have equal metrics. EIGRP can also install eligible unequal-cost paths when the variance setting permits unequal-cost load balancing. These are separate from the question of whether a path qualifies as a feasible successor.

For a basic example, compare the local EIGRP metrics for all available paths. The path with the lowest feasible distance is the primary choice:

Path through R2: FD 30   <-- successor
Path through R3: higher local metric
Path through R4: higher or alternate local metric

What is an EIGRP feasible successor?

A feasible successor is an alternate EIGRP path that passes EIGRP's feasibility condition. It is a prequalified, loop-free backup path.

A feasible successor is normally retained in the topology table, but it is not necessarily installed as an active route in the IP routing table. If the current successor fails, EIGRP can promote an available feasible successor immediately. It does not need to query neighbors for a new route calculation in that situation.

Do not define a feasible successor as simply the second-best route. A path can have the second-lowest total metric and still fail the feasibility condition. Conversely, a path with a worse total local metric can qualify if its neighbor reports a sufficiently low distance to the destination.

Feasible distance and reported distance

Feasible distance

Feasible distance (FD) is the local router's total calculated EIGRP metric for reaching a destination through a particular neighbor. It includes the local cost to reach that neighbor plus the metric the neighbor reports for the destination.

Once EIGRP selects the successor, the successor's FD becomes the reference value for testing alternate paths.

Reported distance

Reported distance (RD) is the metric a neighbor reports for reaching the destination. It is also called advertised distance (AD) in EIGRP discussions.

The term AD can be confusing. In EIGRP route calculations, advertised distance means reported distance. It is not the same as administrative distance, which is a preference value used when different routing sources offer the same prefix.

• RD or advertised distance: Used in the EIGRP feasibility-condition test.
• Administrative distance: Used to compare route sources, such as EIGRP, OSPF, and static routing.

The EIGRP feasibility condition

An alternate route qualifies as a feasible successor only when its reported distance is strictly less than the current successor's feasible distance:

candidate RD < successor FD

The comparison uses the neighbor's reported distance, not the candidate path's full local feasible distance.

The inequality is strict. Equality does not qualify:

Successor FD = 30
Candidate RD = 30

30 < 30 is false
The candidate is not a feasible successor.

The rule provides EIGRP with a loop-free guarantee. A neighbor that reports a distance lower than the current router's successor FD is asserting a route to the destination that is closer, according to EIGRP's metric, than the router's current feasible distance reference.

A candidate can have a worse total local metric than the successor and still qualify. For example, if the successor FD is 30 and an alternate neighbor reports RD 25, the alternate passes the test even if the local cost to reach that neighbor makes the alternate's total FD 40.

Failure of the feasibility condition does not mean the alternate route is permanently unusable. It means only that EIGRP cannot use it as an immediately usable feasible successor. If the successor fails, EIGRP may need to query neighbors and recalculate the route.

Worked five-router topology example

Consider a five-router EIGRP topology. R1 is evaluating the destination prefix 10.0.0.0/24. R5 advertises or originates that destination. R1 learns paths through R2, R3, and R4.

                 R5 -- 10.0.0.0/24
                /  \
               /    \
R1 -------- R2       R4
 \           \
  \           R3

Candidate paths:
R1 -> R2 -> R5
R1 -> R3 -> R2 -> R5
R1 -> R4 -> R5

Assume the path through R2 has a local feasible distance of 30. EIGRP selects it as the successor because it is the best path in this scenario. R1 installs the successor in its routing table.

Now evaluate the alternate paths using the successor FD of 30:

Candidate next hop | Path to destination | Successor FD | Neighbor RD | RD < FD | Role

R2 | R1 -> R2 -> R5 | 30 | Supporting value for successor | Successor selected | Successor

R3 | R1 -> R3 -> R2 -> R5 | 30 | 50 | 50 < 30 is false | Nonqualifying alternate

R4 | R1 -> R4 -> R5 | 30 | 25 | 25 < 30 is true | Feasible successor

Interpreting the example

• R2 path: Its FD is 30, the lowest local metric in the example, so it is the successor. It is installed in the routing table.
• R3 path: R3 reports an RD of 50. Since 50 is not less than the successor FD of 30, this path fails the feasibility condition. It remains an alternate in EIGRP's information but is not a feasible successor.
• R4 path: R4 reports an RD of 25. Since 25 is less than 30, the path qualifies as a feasible successor and is retained as an immediately usable backup in the topology table.

The R4 path could have a higher total local metric than the R2 successor and still qualify. Feasible-successor eligibility depends on the neighbor's RD compared with the successor's FD, not merely on ranking the full path metrics.

Successor versus feasible successor

Characteristic | Successor | Feasible successor

Purpose | Best path used for normal forwarding | Prequalified backup path

Metric relationship | Normally the lowest feasible distance | May have a higher total local metric than the successor

Feasibility requirement | Must be a valid selected route | Must satisfy RD < successor FD

Routing-table presence | Normally installed | Not necessarily installed as an active route

Topology-table presence | Recorded with successor information | Retained as eligible backup information

After successor failure | Fails or is replaced | Can be promoted immediately without neighbor queries

Topology table versus routing table

Information | Topology table | Routing table

Successor | Recorded with its EIGRP metrics and next hop | Normally installed for forwarding

Feasible successor | Retained as a qualifying backup | Usually not shown as the active route

Nonqualifying alternate | May remain as learned topology information | Not installed as a forwarding route

Forwarding use | Database used by EIGRP and DUAL | Table used by the router to forward packets

This distinction explains why a backup can exist in EIGRP output without appearing in show ip route. The topology table is broader than the forwarding table.

What happens when the successor fails?

Failure with a feasible successor

EIGRP normally considers a stable destination passive. If the successor becomes unavailable and a feasible successor is already present, EIGRP can promote that backup locally. The destination does not need a diffusing computation to discover a new path.

Before failure:
Successor       R2
Feasible backup R4

After R1-R2 fails:
Successor       R4

This rapid local replacement is one of the main operational benefits of feasible successors.

Failure without a feasible successor

If the successor fails and no path satisfies the feasibility condition, EIGRP cannot immediately select an alternate as a guaranteed loop-free backup. The destination can enter the active state while EIGRP queries neighbors and waits for replies during route recomputation.

Stuck-in-active is a related troubleshooting condition in which EIGRP does not receive required query replies within the expected process. It is not the normal result when a valid feasible successor is available.

Configuring a simple EIGRP lab

A basic lab requires consistent EIGRP autonomous-system numbering and suitable interface addressing on every router. The exact network statements depend on the topology.

router eigrp <asn>
 network <connected-network> <wildcard-mask>
 no auto-summary

Use the same EIGRP autonomous system number on all routers that should form the intended adjacencies. Configure the appropriate connected networks and interfaces for the selected lab design.

Verification commands

Check neighbors

show ip eigrp neighbors

Confirm that R1 has formed adjacencies with the intended next-hop routers before interpreting route candidates. A missing adjacency means the expected path information may not be present.

Check installed EIGRP routes

show ip route eigrp
show ip route 10.0.0.0

These commands show the EIGRP route or routes installed in the routing table. They help identify the successor and any equal- or unequal-cost paths that are actually being used for forwarding.

Check topology and feasibility data

show ip eigrp topology
show ip eigrp topology 10.0.0.0 255.255.255.0
show ip eigrp topology all-links

Use detailed topology output to find the successor, feasible distance, reported distance, alternate paths, and route state. Software releases can use different output labels or formatting, but the concepts remain FD, RD, successor, and feasible successor.

When reading the output, first identify the current successor FD. Then compare each candidate neighbor's RD with that FD. Do not compare only the candidates' total path metrics, and do not use administrative distance for the feasibility test.

Observe changes during testing

show ip eigrp topology active
debug eigrp packets
debug eigrp fsm

Use these commands only in a controlled lab. Debug commands can produce substantial output and may affect device performance. The active-topology command is useful when investigating a route that is undergoing recomputation.

Common troubleshooting situations

An alternate route is visible but is not an immediate backup

The candidate neighbor's RD may be greater than or equal to the current successor FD. Inspect the destination with show ip eigrp topology 10.0.0.0 255.255.255.0, then compare every candidate RD with the successor FD. The route cannot be called a feasible successor unless RD < FD.

The backup route is absent from the routing table

A feasible successor normally remains in the topology table rather than appearing as an active route in the routing table. Check both show ip eigrp topology and show ip route eigrp before concluding that no backup exists.

The destination enters active state after primary-path failure

This commonly means no feasible successor was available when the successor failed. Use show ip eigrp topology active and review the destination's topology entries and reported-distance values. Restore reachability and verify that neighbors can answer queries. Where appropriate, review the topology or metrics if a prequalified backup is required.

Administrative distance is being used in the feasibility test

This is a terminology error. Review topology output for reported or advertised distance, and review route output separately for administrative distance. Use reported distance in the RD < successor FD test. Administrative distance is relevant to route-source preference, not EIGRP feasibility.

Common misconceptions

• A feasible successor is not merely the second-best route. It must pass the strict feasibility condition.
• Not every alternate path is a feasible successor. An alternate with RD greater than or equal to the successor FD does not qualify.
• A topology-table entry is not automatically a routing-table entry. EIGRP can retain learned paths that are not used for forwarding.
• The feasibility test uses the neighbor's RD. It does not use the candidate's full local FD.
• Feasible-successor eligibility and unequal-cost load balancing are different. The feasibility condition identifies a loop-free backup; variance controls whether eligible unequal-cost paths may be installed for load balancing.
• Administrative distance is not advertised distance. The abbreviations can look similar, but they describe different concepts.

Exam-relevant summary

1. For each destination prefix, EIGRP can learn multiple paths from different neighbors.
2. The successor is normally the path with the lowest feasible distance and is installed in the routing table.
3. The successor's FD is the reference value for feasibility testing.
4. A candidate becomes a feasible successor only when its neighbor-reported distance is strictly less than the successor FD.
5. Equality fails: RD = FD does not qualify.
6. A feasible successor is retained in the topology table and can replace a failed successor without queries.
7. Without a feasible successor, EIGRP may query neighbors and place the destination into the active state.
8. Use EIGRP topology output to compare each neighbor's RD with the current successor FD.

For broader routing fundamentals, review the computer network fundamentals lesson. OSPF learners can also compare this behavior with OSPF route configuration, while remembering that OSPF uses a link-state database rather than EIGRP's topology-table and feasible-successor process.