Advanced distance vector with DUAL: per-destination, loop-free by math, near-instant failover when a backup already exists. Protocol 88, multicast 224.0.0.10, AD 90 internal / 170 external.

The vocabulary

• FD = your total metric to the prefix. RD = what the neighbour reports as its own distance.
• Successor = lowest FD, installed in the RIB. Feasible Successor = loop-free backup already sitting in the topology table.
• Feasibility condition: RD < FD — the neighbour cannot possibly be looping through you. That single inequality is EIGRP’s loop guarantee.

Failure scenarios

• Successor down, FS present → instant promotion, zero queries.
• No FS → route goes Active, queries flood the domain; missing replies turn into Stuck-in-Active. Cure: eigrp stub on spokes, smaller query domains.

Timers & tables

Hello 5/60 s, hold 15/180 s. Neighbor table ≠ topology table ≠ RIB: only the successor reaches the RIB.

Goodies

variance 2                # unequal-cost load balancing (FS only!)
eigrp stub connected      # cut the query domain
show ip eigrp topology    # FD/RD pairs per route