6.5 Enhanced IGRP Operation
6.5.1 Building the neighbor table
In this section, you learn how EIGRP discovers neighbors, discovers routes, chooses routes, and maintain routes when there is a change in the network. This section focuses on the following elements of EIGRP router performance:
  • Building the neighbor table
  • Discovering routes
  • Choosing routes
  • Maintaining routes

Recall that a neighbor table is a table that is maintained by the EIGRP router and that lists adjacent routers. Its purpose is to ensure communication between each of the directly connected neighbors.

Like OSPF, EIGRP routers multicast hello packets to discover neighbor routers. If you recall, adjacent routers are the only ones that can exchange routing information. Each router builds a neighbor table from hello packets that it receives from adjacent EIGRP routers running the same network-layer protocol.

Hello packets are sent out periodically to verify an EIGRP neighbor's availability. On a multi-access, point-to-point connection, and multi-point configurations with bandwidth greater than T1, hellos are sent every five seconds, by default. On an NBMA network, such as multipoint circuits with bandwidth T1 or less, hellos are sent every 60 seconds, by default.

EIGRP maintains a neighbor table for each configured network-layer protocol. Use the show ip eigrp neighbors command to read the table, as shown in the main figure.

The neighbor table includes the following key elements:

  • Neighbor address --- The network-layer address of the neighbor router.
  • Queue count --- Indicates the number of packets waiting in queue to be sent. If this value is constantly higher than zero, then there may be a congestion problem at the router. A zero means that there are no EIGRP packets in the queue.
  • Smooth Round Trip Timer (SRTT) --- Indicates the average time it takes to send and receive packets from a neighbor. This timer is used to determine the retransmit interval (RTO).
  • Hold Time --- The interval to wait without receiving anything from a neighbor before considering the link unavailable. Originally, the expected packet was a hello packet, but in current Cisco IOS software releases, any EIGRP packets received after the first hello will reset the timer.