Simple distance vector routers do not establish any relationship with their neighbors. RIP and IGRP routers merely broadcast or multicast updates on configured interfaces. In contrast, EIGRP routers actively establish relationships with their neighbors as do OSPF routers.

The neighbor table is the most important table in EIGRP. Each EIGRP router maintains a neighbor table that lists adjacent routers. This table is comparable to the adjacency database used by OSPF. There is a neighbor table for each protocol that EIGRP supports.

EIGRP routers establish adjacencies with neighbor routers by using small hello packets. Hellos are sent by default every five seconds. An EIGRP router assumes that, as long as it is receiving hello packets from known neighbors, those neighbors and their routes remain viable or passive. By forming adjacencies, EIGRP routers do the following:

• Dynamically learn of new routes that join their network
• Identify routers that become either unreachable or inoperable
• Rediscover routers that had previously been unreachable

The following fields are found in a neighbor table:

• Neighbor address – This is the network layer address of the neighbor router.
• Hold time – This is 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) – This is 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) – This is the number of packets waiting in a queue to be sent. If this value is constantly higher than zero, there may be a congestion problem at the router. A zero means that there are no EIGRP packets in the queue.
• Sequence Number (Seq No) – This is the number of the last packet received from that neighbor. EIGRP uses this field to acknowledge a transmission of a neighbor and to identify packets that are out of sequence. The neighbor table is used to support reliable, sequenced delivery of packets and can be regarded as analogous to the TCP protocol used in the reliable delivery of IP packets.

Interactive Media Activity Crossword Puzzle: EIGRP Concepts and Terminology When the student has completed this activity, the student will understand the different EIGRP concepts and terminology.