5-6 Verifying and Troubleshooting EIGRP

The following three commands are used to verify and troubleshoot EIGRP:

• show ip route
• show ip protocols
• show ip eigrp neighbors
• show ip eigrp topology
• debug eigrp packets and debug ip eigrp notifications

The show ip route command has been covered in the previous section and earlier in this chapter. Eventually a complete and correct routing table across the network is the best verification of a routing table. The other two commands are covered below.

Using show ip protocols command to verify and troubleshoot EIGRP 

The show ip protocols command helps verify routing protocols running on the router. An example of the output of this command from RouterA of our network is shown below:

The show ip protocols commands shows the operational information for EIGRP. From the above output you can gather that the EIGRP AS is 10 and it is advertising the 192.168.1.0 and 192.168.4.0 networks. You can also learn that it is using default metrics (K1 and K3) and the maximum hop count has been configured as 255. The output also shows that auto summary is disabled and maximum path is set to 6. As before, this output helps confirm the configuration of EIGRP.

Using show ip eigrp neighbors command to verify adjacencies 

It is important to see which routers EIGRP has formed adjacencies with and how stable the adjacencies are. The show ip eigrp neighbors command helps do this. The output from RouterE in our network is shown below:

The various fields in the output are discussed below:

• The H field indicates the order in which the neighbors were discovered.
• The Address indicates the IP address of the neighbor.
• The Interface indicates the interface via which the neighbor is reachable.
• The hold time shows how long this router will wait for a Hello packet to arrive from the neighbor.
• The uptime indicates the time since the adjacency was established.
• The SRTT or the smooth round-trip timer indicates the time it takes for a round-trip from this router to its neighbor and back. This value determines the amount of time router will wait after a multicast for a reply from this neighbor. If a reply isn’t received in time, the router will switch to using unicasts.
• The RTO or Retransmission Time Out field indicates the amount of time EIGRP waits before retransmitting a packet from the retransmission queue to a neighbor.
• The Q value indicates whether there are any messages in the queue waiting to be sent to the particular neighbor. Consistently high values would indicate a problem in communication between the neighbors.
• The Seq, or sequence, field indicates the sequence number of the last update from the neighbor. As you know, EIGRP uses sequence numbers to keep track of updates and replies received to updates.

The various fields in the output help keep track of EIGRP adjacencies and their health. For example, high value in the SRTT, RTO or Q fields would indicate problems in communication.

Using show ip eigrp topology command to verify and troubleshoot EIGRP

Since EIGRP stores all routes learned in a topology table, looking at the topology table can give you indications about the functioning of EIGRP as well as the stability of the network. The show ip eigrp topology command shows the topology table. An example from RouterC in our network is shown below:

Notice that a P precedes all routes. This means that the routes are in a passive state. A route in passive state means it is currently known about and stable. On the other hand, a route in active state (preceded by A) means that the route has lost the route and is searching for a replacement. Each route also shows the Feasible Distance (FD) and the next hop address towards the destination. The two numbers in the parenthesis are the feasible and advertised distances. For routes such as 192.168.4.0/24 notice that there are two next hops. The first one is the successor and will be seen in the routing table of the router while the second one is the feasible successor. The second route will be used if the first one is lost. If the feasible distance of two routes is displayed as the same, EIGRP will load balance between the two routes.

Using debugs to troubleshoot EIGRP

As with RIP, there are some debugs for EIGRP and then help troubleshoot. The most important of all EIGRP debugs is the debug eigrp packets. Using this debug you can see EIGRP packets as they come into and go out of a router. On a router where EIGRP is working normally and has established adjacencies, you should only see hello packets coming in and going out in the debug output as shown below:

In the above output you can see hello packets being sent out every interface with the AS number shown. You can also see hello packets received from neighbors 192.168.2.2 and 192.168.6.5. The received packets also show the AS number. If the AS numbers in the updates do not match the AS number configured in the router, the packets will be dropped.

Another important debug for EIGRP is the debug ip eigrp notifications. During normal operations, you will not see any outputs for this debug but if there is change in the network. To generate the output below, I shutdown the s0/0 interface on RouterE, causing RouterC to loose the adjacency with that router.

When the s0/0 interface on RouterE is brought back up, the following debugs are seen on RouterC:

You can use this debug to verify that your network is stable and there are no constant changes in the network. No output for this debug command is good news!