• Feasibility Condition:
    • Used for selecting backup paths.
    • Pre-computed during DUAL
      • Advantages
        • Sub-second re-convergence when successor fails.
        • Fault isolation.
          • No need to speak with neighbors about additional paths.
        • Unequal cost distribution.
    • What the process is doing:
      • Find best path, hold onto local metric.
      • Find second best path.
        • Any router with metric lower than mine, they’re closer to destination.
        • Any router with metric equal or higher than mine, forget about them.

  • Successor – Best Path
  • Feasible Distance – Metric of Successor
  • Reported Distance – Metric of neighbors
  • Feasible Successor – Backup paths that meet Feasibility Condition
  • Only Feasible Successors can be used for unequal cost load balancing


  • If no FS:
    • Sends route into Active state
    • Sends QUERY to all neighbors
    • Reconverges from neighbor REPLY
  • With FS:
    • No route Active
    • FS promoted to Successor
    • QUERY not generated
    • Results in sub-second convergence

Bandwidth and Delay

  • Bandwidth
    • DUAL uses the lowest bandwidth along a path to a destination.
    • By prefix.
  • Delay
    • DUAL uses delay by adding up hop by hop to destination.
    • Can be modified under interface with ‘delay’ command.

Unequal Load Balancing:

  • Only Feasible Successors allowed.
  • Controlled with Variance command.
    • FD * variance > FS, load balancing happens.
  • Commands:
    • router eigrp <name>
      • address-family ipv4 autonomous-system <AS #>
      • topology base
      • variance <variance multiplier>

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