6.3 EIGRP Components
6.3.2 EIGRP tables
DUAL can select alternate routes based on the tables kept by EIGRP. By building these tables, every EIGRP router can track all the routing information in an AS, not just the "best" routes.

The following sections examine the neighbor table, the routing table, and the topology table in detail and provide an example of each. In addition, we will look at the various packet types used by EIGRP to build and maintain these tables.

The Neighbor Table
The most important table in EIGRP is the neighbor table (refer to Figure ). The neighbor relationships tracked in the neighbor table is the basis for all the EIGRP routing update and convergence activity.

The neighbor table contains information about adjacent neighboring EIGRP routers. Whenever a new neighbor is discovered, the address of that neighbor and the interface used to reach it are recorded in a new neighbor table entry.

A neighbor table is used to support reliable, sequenced delivery of packets. One field in each row of the table includes the sequence number of the last packet received from that neighbor. EIGRP uses this field to acknowledge a neighbor's transmission and to identify packets that are out of sequence.

As shown in Figure , an EIGRP neighbor table includes the following key elements:

  • Neighbor address (Address) - The network-layer address of the neighbor router.
  • Hold time (Hold Uptime) - 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.
  • Smooth Round-Trip Timer (SRTT) - The average time that it takes to send and receive packets from a neighbor. This timer is used to determine the retransmit interval (RTO).
  • Queue count (Q Cnt) - 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.

Note that an EIGRP router can maintain multiple neighbor tables, one for each PDM running (e.g., IP, IPX, and AppleTalk as shown in Figure ). A router must run a unique EIGRP process for each routed protocol.

The Routing Table
The routing table contains the routes installed by DUAL as the best loop-free paths to a given destination as shown in Figure . EIGRP will maintain up to four routes per destination. These routes can be of equal or unequal cost. EIGRP routers maintain a separate routing table for each routed protocol.