6.5 Enhanced IGRP Operation
6.5.2 Discovering routes

The neighbor establishment and discovering routes processes occur at the same time in EIGRP. A high-level description of the process is shown in Figures - and defined here:

  1. A new router (router A) comes up on the link and sends out a hello through all interfaces.
  2. Routers receiving the hello reply with update packets that contain all the routes they have in their topology tables, except those learned through that interface (the split horizon process requires that information not be sent back in the direction it was received). In addition, these update packets have the Init bit set, indicating that this is the initialization process.
    An update packet includes information about the routes a neighbor is aware of, including the metric that the neighbor is advertising for each destination.
  3. Router A replies to each neighbor with an Ack (acknowledgment) packet, indicating that it received the update information.
  4. Router A puts all update packets in its topology table.
    The topology table includes all destinations advertised by neighboring (adjacent) routers. It is organized such that each destination is listed, along with all the neighbors that can get to the destination, and their associated metrics.
  5. Router A then exchanges update packets with each of its neighbors.
  6. Upon receiving the update packets, each neighbor router sends an Ack packet back to router A.
    When all updates are received, the router is ready to choose the primary and backup routes to keep in the topology table.