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Chapter 6
Troubleshooting TCP/IP Connectivity
You can see from this information when routes are removed from the
route table and they are no longer advertised. Once the route is advertised to
the router, it inserts the route back into the route table and commences
advertisement.
Typical Problems
Some of the typical problems with EIGRP are the loss of neighbor adjacen-
cies, lost routes in earlier versions of IOSTM, and lost default gateways.
Neighbor failures can be attributed to link failures, just as much as they
can be attributed to software problems. If a neighbor relation has problems
establishing, use the proper debug command to see what is occurring
between both routers.
When troubleshooting an EIGRP problem, it is always a good idea to get
a picture of the network. The most relevant picture is provided by the show
ip eigrp neighbors
command. This command shows all adjacent routers
that share route information within a given autonomous system. If neighbors
are missing, check the configuration and link status on both routers to verify
that the protocol has been configured correctly.
If all neighbors are present, verify the routes learned. By executing the show
ip route eigrp
command, you gain a quick picture of the routes in the route
table. If the route does not appear in the route table, verify the source of the
route. If the source is functioning properly, check the topology table.
The topology table is displayed by using the show ip eigrp topology
command. If the route is in the topology table, it is safe to assume that there
is a problem between the topology database and the route table. There must
be a reason why the topology database is not injecting the route into the
route table.
Other commands, such as show ip eigrp traffic, can be used to see
whether updates are being sent. If the counters for EIGRP input and output
packets don't increase, no EIGRP information is being sent between peers.
The show ip eigrp events command is an undocumented command.
This command displays a log of every EIGRP event--when routes are
injected and removed from the route table, and when EIGRP adjacencies
reset or fail. This information can be used to see whether there are routing
instabilities in the network.
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