To complete the review of prerequisite CCNA-level EIGRP knowledge, this section looks at a few of the internals of EIGRP. Some of the facts listed here simply need to be memorized, whereas other topics will be discussed in more detail later.
EIGRP follows three general steps to add routes to the IP routing table, as follows:
Step 1. Neighbor discovery: EIGRP routers send Hello messages to discover potential neighboring EIGRP routers and perform basic parameter checks to determine which routers should become neighbors.
Step 2. Topology exchange: Neighbors exchange full topology updates when the neighbor relationship comes up, and then only partial updates as needed based on changes to the network topology.
Step 3. Choosing routes: ...