EIGRP neighbors

Routers that run EIGRP must become neighbors before exchanging routing information. To dynamically discover neighbors on the directly attached networks, EIGRP routers use the multicast address of to send Hello packets every couple of seconds. To become neighbors, the following parameters must match on both routers:

  • ASN (Autonomous System Number)
  • subnet
  • K values (components of metric)

Let’s examine each of these parameters in detail:

  • ASN (Autonomous System Number) – EIGPR uses the concept of autonomous systems. An autonomous system is simply a group of EIGRP-enabled routers that should become EIGRP neighbors and exchange routes. An AS is defined on a router by using the router eigrp AS_NUMBER global configuration command. In order to become neighbors, all routers must be configured with the same AS number.
  • subnet – interfaces on both routers must be in the same subnet.
  • K values – each router must be configured with the same K values used to calculate the metric. By default, the only parameters used to calculate an EIGRP metric are bandwidth and delay. If a network administrator wants to use other parameters (load and reliability) to calculate a metric, the change has to be made on both routers. Otherwise, the mismatched K-values will prevent routers to become neighbors.

Consider the following example:

eigrp neighbor discovery

We have a simple network of two routers. Both routers are running EIGRP, with the ASN of 1. Because the interfaces on both routers are in the same subnet and the K-values match, the routers will become neighbors:

eigrp new adjacency

The information about EIGRP neighbors are stored in the neighbor table. We can display the content of this table by running the show ip eigrp neighbors command:

show ip eigrp neighbors

Here is a brief description of each column

  • H – the sequential numbering of established neighbor adjacencies. The first neighbor will have a value of 0, the second neighbor a value of 1 and so on.
  • Address – the IP address of the EIGRP neighbor.
  • Interface – the interface on the local router on which the Hello packets were received.
  • Hold (sec) – the holddown timer. It specifies how long will EIGRP wait to hear from the neighbor before declaring it down.
  • Uptime – the time in hours:minutes: seconds since the local router first heard from the neighbor.
  • SRTT (ms)Smooth round-trip time. The time it takes to send an EIGRP packet and receive an acknowledgment from the neighbor.
  • RTORetransmission timeout in milliseconds. This is the time that EIGRP will wait before retransmitting a packet from the retransmission queue to a neighbor.
  • Q Cnt – the number of EIGRP packets (Update, Query or Reply) in the queue that are awaiting transmission. Should be 0.
  • Seq Num – the sequence number of the last update, query, or reply packet that was received from the neighbor.
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