EIGRP - Feasible Successors and the DUAL Algorithm

When a successor route fails in EIGRP, the protocol will look for a Feasible Successor (FS). If an FS exists, it will be promoted to the successor route immediately without initiating the Diffusing Update Algorithm (DUAL) process. This mechanism allows EIGRP to achieve fast reconvergence.

However, what if a third route exists with a lower distance than the FS but which hadn't been originally considered because it failed the Feasibility Condition (FC)? Well, this is what would happen:

Key points:

  1. Successor and Feasible Successor (FS):

    • Route 1: Original successor.
    • Route 2: Feasible successor calculated by EIGRP.
    • Route 3: Another route with a lower distance but failed the Feasibility Condition (FC) check.

  2. Failover Process:

    • If Route 1 fails, EIGRP will immediately place Route 2 (FS) in the routing table without running DUAL.
    • Route 3 will not be considered unless DUAL is triggered.

  3. What triggers a DUAL Process?:

    • Loss of the successor route without an available FS.
    • Significant topology change with no immediate FS.
    • Receiving a DUAL query from a neighbor.
    • Route metric change that invalidates the current FS.
    • No FS found during a route update.

  4. When will Route 3 be checked using the FC to be considered as an FS?:

    • EIGRP determines new FS as part of its normal background operations, not during DUAL computation.

Route 3 will only be evaluated as a candidate successor if the DUAL process is triggered under specific conditions. This ensures stable and efficient routing within the network.

Links:

https://networklessons.com/eigrp/introduction-to-eigrp/

https://forum.networklessons.com/t/introduction-to-eigrp/881/210?u=lagapidis