Content

| 5.3 |  | EIGRP Components |
| 5.3.2 |  | 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, the various packet types used by EIGRP to build and maintain these tables will be investigated.

The Neighbor Table
The most important table in EIGRP is the neighbor table. 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.

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. However, 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, for example, IP, IPX, and AppleTalk. A router must run a unique EIGRP process for each routed protocol.

| Interactive Media Activity Point and Click: EIGRP Neighbor Table Upon completion of this activity, the student will be able to identify the key elements of the EIGRP neighbor table.