Introduction to EIGRP Neighbors

EIGRP enabled routers to establish neighbour adjacencies with other EIGRP enabled routers by exchanging EIGRP Hello packets. Without establishing neighbor adjacencies routers cannot send or receive any updates. Using the “show ip eigrp neighbors” command we can examine the neighbors table and verify EIGRP adjacencies. Figure 1 illustrates the output of the “show ip eigrp neighbors” command.

We can see the IPv4 address for each adjacent router and the interface that this router uses to reach that EIGRP neighbors. Using this topology, each router has two adjacent neighbors listed in the neighbors table. The output also includes the following:

  • H column– This field lists the neighbors in the order that they were learned. The first neighbor will have a value of 0, the second neighbors a value of 1 and so on.
  • Address– This is the IPv4 address of the adjacent neighbors.
  • Interface– This is the local interface on which this Hello packet was received.
  • Hold– This field specify how long EIGRP will wait to hear from the neighbour before declaring it down. When a Hello packet is received, the time value is reset to the maximum hold time for that interface, and then again counts down to zero. If zero is reached, the neighbour is considered down.
  • Uptime– This is the time since this neighbour was added to the neighbour table. The time in hours:minutes: seconds.
  • Smooth Round Trip Timer (SRTT) –The time it takes to send an EIGRP packet and receive an acknowledgement from the neighbour.
  • Retransmission Timeout (RTO) – The time that EIGRP will wait previous to retransmitting a packet from the retransmission queue to a neighbour for reliable EIGRP packets.
  • Queue Count– This values always be zero, if the value is more than zero, then EIGRP packets wait to be sent.
  • Sequence Number– The sequence number is used to track updates, queries, and reply packets.

This command is useful for verifying and troubleshooting EIGRP problems. We can check the neighbour IP address in the adjacencies table. We can also use the “show ip interface brief” command to verify the interface state. If the interface is an active state then we can try pinging the IPv4 address of the neighbour. If the ping not received, it means that the neighbour interface is down and must be activated. If the ping is successful and doing well then we should check that the EIGRP autonomous system number must be the same.