You want to log EIGRP neighbor state changes.
To enable the logging of EIGRP neighbor state changes, use the eigrp log-neighbor-changes configuration command:
Router1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#router eigrp 55
Router1(config-router)#eigrp log-neighbor-changes
Router1(config-router)#end
Router1#
When a neighbor relationship is lost, you also lose all of the routing entries for that neighbor. The effects of this lost routing information are often felt throughout the network. Therefore, it can be extremely useful to have a good log of neighbor change events for troubleshooting strange intermittent network problems. This feature also gives you a good way of looking for faults on links that don't have a way of telling you about loss of connectivity.
Two important examples of this are tunnels and LAN extensions. In many cases, when the network breaks and brings down a tunnel, the two tunnel end points are unable to see the problem. Similarly, in a LAN extension service, the two end point routers are both connected to Layer 2 LAN switches that are then bridged to one another through another medium, such as ATM. The problem in this case is that the intermediate network between the switches can break and neither router will see a problem because they are both connected to an active switch port. It's also important to note that EIGRP neighbor relationships can break just because of noisy or congested links.
Whatever the cause, one of the easiest ways to detect a connectivity problem in the hidden network is to configure EIGRP between the routers via this link. In some cases, this will be done on a separate EIGRP process ID number to make it easier to differentiate between normal network topology changes and these hidden network faults. If you log EIGRP neighbor changes and configure the routers to send their SYSLOG events to a central fault management server (as discussed in Chapter 18), you can get an instant alarm on these types of problems.
The log messages show not only that the neighbors have changed, but they also give you an indication of why they changed state:
Oct 2 22:00:38: %DUAL-5-NBRCHANGE: IP-EIGRP 55: Neighbor 172.25.2.1 (Serial0.1) is up: new adjacency Oct 2 22:03:23: %DUAL-5-NBRCHANGE: IP-EIGRP 55: Neighbor 172.25.2.1 (Serial0.1) is down: summary configured Oct 2 22:03:23: %DUAL-5-NBRCHANGE: IP-EIGRP 55: Neighbor 172.25.2.1 (Serial0.1) is up: new adjacency Oct 2 22:04:14: %DUAL-5-NBRCHANGE: IP-EIGRP 55: Neighbor 172.25.2.1 (Serial0.1) is down: manually cleared Oct 2 22:04:19: %DUAL-5-NBRCHANGE: IP-EIGRP 55: Neighbor 172.25.2.1 (Serial0.1) is up: new adjacency Oct 2 22:07:26: %DUAL-5-NBRCHANGE: IP-EIGRP 55: Neighbor 172.25.2.1 (Serial0.1) is down: peer restarted Oct 2 22:07:27: %DUAL-5-NBRCHANGE: IP-EIGRP 55: Neighbor 172.25.2.1 (Serial0.1) is up: new adjacency Oct 2 22:30:06: %DUAL-5-NBRCHANGE: IP-EIGRP 55: Neighbor 172.25.2.1 (Serial0.1) is down: holding time expired Oct 2 22:30:38: %DUAL-5-NBRCHANGE: IP-EIGRP 55: Neighbor 172.25.2.1 (Serial0.1) is up: new adjacency
In this example, we have shown four different reasons for EIGRP to reset its neighbor relationships. Of these, only the last one, holding time expired, is likely to indicate a network fault.
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