EIGRP: Router Adjacencies

EIGRP discovers neighboring routers participating in EIGRP dynamically or manually.

Dynamic neighbor discovery is done by sending EIGRP HELLO packets to the destination multicast address of or FF02::A.  This occurs immediately when an interface begins participating in EIGRP.

Static or Manual neighbor relationships are configured local on the router, and unicast messages are sent to the configured neighbor address to establish a neighborship.

You either have all neighbors dynamically configured to find neighbors or you statically configure all your neighbors…setting them manually turns off the multicast EIGRP traffic so you have to choose one or the other.

Neighborships don’t automatically establish if they’re listening to the multicast traffic.  Neighbor relationships requires an agreement of a set of parameters before a neighborship is formed.

  • EIGRP Authentication Parameters (if configured)
  • EIGRP K-Values
  • EIGRP Autonomous System (AS) Number
  • Use of primary addresses for EIGRP neighbor relationships
  • Use of common IP network address on single subnet

If any of  these parameters differ between neighbors they will not form neighbor relationships.

Hello and Hold timers do not need to match to form a neighbor relationship.

Hold time defines the max time the router should wait to receive subsequent valid EIGRP packets from a neighbor.  If the hold time expires the neighbor is declared unreachable and DUAL is informed of the loss of the neighbor.

Since Hello timers don’t need to match to form a neighbor adjacency this could result in neighborship flaps.  So it’s important to try and match these parameters up as well.




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