EIGRP neighbor and debug EIGRP

EIGRP neighbor and debug EIGRP neighbor relationship issues with the use of debug commands

EIGRP protocol forms the neighbor relationship with the directly connected router before sharing the routing table with the other router however to form the neighbor relationship, some values should matched otherwise neighbor relationship will not work and routers will not share the routing table.

To form a successful neighbor relationship, some of the values between two neighbor routers should match.

Subnet – Both routers should be on the same subnet to form a successful neighbor relationship

K-Values – K values should also match.

AS number – AS number should match between routers.

There are some configurations as well that can interrupt the proper neighbor relationship.

We can check the neighbor table by using the following command.

R1#show ip eigrp neighbors

We can see in the image below that neighbor information is available with this command.

In the output, we can see the IP address assigned to the neighbor and the interface which is used by the neighbor to connect. There are other details available that can help us to troubleshoot the issues with neighbor relationships.

'Show IP neighbor details' show extra information about the neighbor like the version of the IOS etc.

Following is the output of the command ‘show ip neighbor details’

We can check the traffic sent between neighbors by using the command ‘show ip eigrp traffic’

Below is the output of the 'show ip eigrp traffic command, we can see the numbers of hello packets sent and received.

We can see lots of information including the updates sent and received between routers.

Debug eigrp packets command

Debug eigrp packets allow us to troubleshoot the issues with the neighbor relationship, we can use this command to see how the hello packets are sent and received.

The following image shows the output of the ‘debug eigrp packets’ command.

We can see the communication happening between two routers with a successful neighbor relationship, now, we will shut down the interface of one of the routers to see what the output of the command looks like.

As the other router is down, we can see in the following image that the neighbor router is not sending any hello packets however working router is sending the hello packets regularly.

Debug EIGRP neighbors ‘Debug eigrp neighbors’ command can be very useful in fixing the neighbor relationship issues as we have now a lot more information available in this command.

While using the ‘debug eigrp packets’ command we were able to see the information about the packets however ‘debug eigrp neghbors’ command output shows what exactly happened to the neighbor router.

Below is the output of the command when we shut down the interface on the other router, this command has clearly stated that the hold time timer has expired and a peer is going down.