CCNA EIGRP: Understanding Feasible Successors and Feasibility Condition

EIGRP's speed and efficiency come from its ability to identify loop-free backup routes instantly. Unlike RIP or OSPF, EIGRP does not need to recompute the entire topology when a primary route fails. Instead, it relies on feasible successors—pre-calculated backup routes stored in the topology table. Understanding the feasibility condition is essential for CCNA candidates and production network engineers who need to predict convergence behavior and troubleshoot EIGRP path selection.

What is a Successor and Feasible Successor?

In EIGRP, a successor is the next-hop neighbor that offers the lowest-cost path to a destination. This is the route that appears in the routing table. A feasible successor is an alternative next-hop neighbor that can reach the same destination with a metric lower than the current successor's feasible distance, making it a guaranteed loop-free backup.

The key difference: a successor is active; a feasible successor is passive but ready. When the successor fails, EIGRP immediately promotes a feasible successor to successor status without recomputing the topology. This is why EIGRP converges so quickly—there is no waiting for SPF recalculation.

The Feasibility Condition

The feasibility condition is the mathematical rule that guarantees a loop-free backup route. It states: a neighbor can be a feasible successor if its advertised distance (AD) to the destination is strictly less than the current successor's feasible distance (FD).

Feasibility Condition: Neighbor AD < Current Successor FD

Example:
- Successor via Router A: FD = 2000
- Router B advertises: AD = 1500
- Is B a feasible successor? 1500 < 2000? YES
- Router C advertises: AD = 2500
- Is C a feasible successor? 2500 < 2000? NO

This condition works because if a neighbor's AD is less than the current FD, that neighbor cannot be using the current router in its path to the destination. Therefore, no loop can form. The feasible distance for any feasible successor will always be lower than the current successor's FD, ensuring a loop-free topology.

Why This Matters for Convergence

When a successor fails, EIGRP does not send out queries to neighbors. Instead, if a feasible successor exists, it is immediately used. The router updates its routing table and continues forwarding traffic with zero query overhead. This is called a local repair or fast failover.

• →Successor fails -> FD recalculated
• →Feasible successor exists and meets condition -> promoted to successor immediately
• →No topology recomputation, no queries sent, no convergence delay
• →If no feasible successor exists -> queries sent, waiting for responses, slower convergence

Practical Topology Table Example

Router# show ip eigrp topology 10.0.0.0/24
EIGRP Topology Entry for AS(1)/ID(192.168.1.1)
  State is Passive
  Successor(s) (via 192.168.2.1), FD is 2000
    Reported Distance is 1500
  Feasible Successor(s)
    via 192.168.3.1 (Reported Distance 1800)
      FD is 2800
    via 192.168.4.1 (Reported Distance 1900)
      FD is 2900

Both feasible successors have AD < 2000 (current FD)
1800 < 2000? YES
1900 < 2000? YES
Both qualify as feasible successors.

In this example, the primary successor via 192.168.2.1 has a feasible distance of 2000. Both alternative neighbors meet the feasibility condition because their advertised distances (1800 and 1900) are less than 2000. If the primary link fails, EIGRP will use the feasible successor with the lowest FD (2800) without any query process.

Common CCNA Exam Pitfalls

• →Confusing advertised distance with feasible distance: AD is what the neighbor reports; FD is the metric to reach that neighbor plus the neighbor's AD
• →Thinking a feasible successor is chosen based on lowest FD alone: it must first meet the feasibility condition
• →Assuming all neighbors are feasible successors: only those with AD < current FD qualify
• →Forgetting that a feasible successor must be passive: active neighbors cannot serve as backups until the topology stabilizes