EIGRP Hello and Hold Timers

The Hello and Holddown timers can be a bit tricky to understand if you mostly work with, say, OSPF for example. So let's get down to it.

Both timers are defined under an interface. In Classic Mode it is directly under the interface proces, whereas under the Named Mode it will be defined under the af-interface section of the EIGRP process.

The Hello timer defines how often the local router will send out a Hello packet. This information is not advertised to the neighbor and the Hello timer does not need to match between the neighbors for EIGRP to form adjacencies.

The Holddown timer defines how long the neighbor router will wait for a Hello packet from the local router. The way this works is that the Holddown is advertised in the Hello packet, so that the local router can instruct the neighbor how long it should wait for a Hello packet before tearing down the adjacency.

Hold Time advertised in an EIGRP Hello packet

So, because the Hello timer is locally significant only and the Holddown is the timeout for the local routers adjacency on the neighboring router, there is no need for the timers to match between EIGRP neighbors - unlike OSPF where these timers must match.

Note: I do believe the best-practice recommendation would be to have the timers match on the peering devices - but it is a technical possibility to have them configured differently between the adjacent routers.