EIGRP Basics - Hello Packets

Vocab:
IGRP: Interior Gateway Routing Protocol
EIGRP: Enhanced Interior Gateway Routing Protocol
AS: Autonomous System - a group of routers share an AS so they know they are part of a routing group or system and should share routes and form relationships within that group. A router can be a member of multiple AS's but must inject routes between them if the two groups wish to share common routes.

Cisco loves to ask questions on it's own proprietary protocols, so don't skip over something just because it isn't the most common protocol out there. (although it looks like IGRP is getting phased out)

EIGRP Hello Packets:

EIGRP sends hello packets to the multicast address 224.0.0.10 in order to create and maintain Neighbor relationships. These are not "reliable" packets and so do not expect an acknowledgement. When a router receives an EIGRP hello packet, it first checks three items. Assuming it is configured with:

1. The same AS
2. The Primary IP address is in the same subnet
3. The metric-calculation constants match (how it determines route cost/metrics)

Then a neighbor adjacency relationship will be established.

Over a fast link (greater than T1) Hello packets are sent every 5 seconds (default). Over a slow link (T1 or slower) Hello packets are sent every 60 seconds (default).

Hello packets include the hold time. This is the time a router waits, and if it doesn't receive any EIGRP packets from the neighbor switch, flushes the routes and attempts to re-converge deciding the neighbor is down. Hold time, by default is 3 times the hello interval. This means fast links have a hold time of 15 seconds, while slow links are 180 seconds. Remember that changing the hello time does NOT automatically change the hold time, you need to remember to change both.

Issue the command show ip eigrp neighbors to see if your hello packets have formed an adjacency.