EIGRP Neighbor Relationships

Unlike IGRP, EIGRP must establish neighbor relationships before updates are sent out. When an EIGRP process is configured on the router, the router begins to exchange EIGRP hello packets over the multicast address of 224.0.0.10. Neighbor relationships form between routers when they receive each other's hello packet. Over LAN broadcast media such as Ethernet, Token Ring, or FDDI, the hello packets are sent every 5 seconds. Over WAN multipoint interfaces with a bandwidth of T1 or greater, and over point-to-point sub-interfaces, the hello packets are also sent out every 5 seconds. WAN multipoint interfaces with a bandwidth of T1 or lower are considered to be low-bandwidth interfaces, and the hello packets are sent out every 60 seconds.

Aside from the hello time, there is also a notion of a hold time. The hold time tells the router the maximum time that it will wait to reset a neighbor if hello packets are not received. In other words, if the hold time expires before a hello packet is received, the neighbor rela-tionship will be reset. The default value of the hold time is three times the hello time. This means that in the LAN broadcast media where the hello time is 5 seconds, the hold time will be 15 seconds, and the slow WAN interfaces with a hello time of 60 seconds will have a default hold time of 180 seconds. Keep in mind that you can configure the hello and hold times. Certain conditions must be met before EIGRP routers consider establishing a neighbor relationship:

• The receiving router compares the source address of the hello packet with the IP address of the interface where the packet was received, to ensure that they belong to the same subnet.

• The receiving router compares the K constant values of the source router to its own, to make sure that they match.

• The receiving router must be within the same autonomous system number as the source router.

Example 6-1 shows the output of the show ip eigrp neighbor command when the neighbor relationship is fully established.

Example 6-1 show ip eigrp neighbor Command Output

 Router_1#  show ip eigrp neighbor  IP-EIGRP neighbors for process 1 H   Address    Interface   Hold Uptime   SRTT   RTO  Q  Seq                                         (sec)  (ms) Cnt Num 1   5.5.5.4       Et0        11 00:00:22    1  4500  0  3 0   192.168.9.5   Et1        10 00:00:23  372  2232  0  2 

The explanations of the heading of the output are as follows :

• H ‚ The list of the neighbors in the order in which they are learned.

• Address ‚ The IP address of the neighbors.

• Interface ‚ The interface from which the neighbors are learned.

• Hold ‚ The hold timer for the neighbor. If this timer reaches 0, the neighbor relationship is torn down.

• Uptime ‚ The timer that tracks how long this neighbor has been established.

• SRTT (Smooth Round Trip Time) ‚ The average time in which a reliable EIGRP packet is sent and received.

• RTO (Round Trip Timeout) ‚ How long the router will wait to retransmit the EIGRP reliable packet if acknowledgment is not received.

• Q Count ‚ The number of EIGRP packets waiting to be sent to the neighbor.

• Sequence Number ‚ The sequence number of the last EIGRP reliable packets being received from the neighbor. This is to ensure that packets received from the neighbor are in order.