Viewing Route Updates

Viewing Route Updates
Viewing the routing updates that your router is sending and receiving is an invaluable tool. The
following debug commands give you a step-by-step account of what is happening between your
router and its neighbors:
Router#debug ip igrp events
IGRP event debugging is on
Router#
15:34:08: IGRP: received update from 10.10.10.1 on Serial2/0.1
15:34:08: IGRP: Update contains 0 interior, 1 system, and 0 exterior routes.
15:34:08: IGRP: Total routes in update: 1
15:34:55: IGRP: sending update to 255.255.255.255 via Serial2/0.1 (10.10.10.2)
15:34:55: IGRP: Update contains 0 interior, 1 system, and 0 exterior routes.
Interior Gateway Routing Protocol 117
15:34:55: IGRP: Total routes in update: 1
15:34:55: IGRP: sending update to 255.255.255.255 via Loopback0 (192.168.24.1)
15:34:55: IGRP: Update contains 0 interior, 2 system, and 0 exterior routes.
15:34:55: IGRP: Total routes in update: 2
The debug ip igrp events command allows you to view the routing updates sent to the router
every 90 seconds by its neighbors. It also allows you to view the routing updates that the router sends
to its neighbors every 90 seconds. The routing updates sent by the router to its neighbors contain the
destination address (the broadcast address 255.255.255.255 or a unicast address, due to the use of
the neighbor command), interface, a summary of the types of routes, and the number of routes sent.
The routing updates received by the router from its neighbors include the same information, except
the destination address is replaced with the source address.
If the data contained in the debug ip igrp events doesn’t give you enough information,
there is another debug command that will provide you with more information about the routes
contained in the routing update. Enter the following command in privileged EXEC mode:
Router#debug ip igrp transactions
IGRP protocol debugging is on
Router#
15:36:21: IGRP: sending update to 255.255.255.255 via Serial2/0.1 (10.10.10.2)
15:36:21: network 192.168.24.0, metric=501
15:36:21: IGRP: sending update to 255.255.255.255 via Loopback0 (192.168.24.1)
15:36:21: network 20.0.0.0, metric=7382
15:36:21: network 10.0.0.0, metric=6882
15:37:07: IGRP: received update from 10.10.10.1 on Serial2/0.1
15:37:07: network 20.0.0.0, metric 7382 (neighbor 501)
The debug ip igrp transactions command provides detailed information about the
routes contained in the routing update. As you can see in the debug output, the debugging of
IGRP transactions contains all networks that were in the update, as well as their metrics.
Updates received by the router also contain the neighbor’s metric. Recall that the neighbor’s
advertised metric is used in conjunction with the variance command to avoid the trap of load
balancing by using a potential routing loop.
Both of the debug commands discussed in this section can be entered to display the detailed
information about the routes, as well as display a summary of the types of routes. This makes it
convenient to view all the information about the updates that the router is sending and receiving.
In order for you to be able to view debug output, you must have logging enabled. Logging
will send the debug messages to the destination that you state in the logging command. In
order to enable logging, you may need to enter the logging on command in global configuration
mode. After enabling logging, you can specify where to send the messages. The following
is a list of some of the most common destinations for a message to be sent:
 Using a host name or IP address will send the messages to a Syslog server.
 buffered will send the messages to the router’s buffer.
 console will send the messages to the router’s console.
118 Chapter 4  IGRP and EIGRP
These destinations are not the only places logging can be set up for. In order to configure the
router to send messages to one of these destinations, you will need to enter the logging command
followed by one of the destinations in global configuration mode.
After enabling debugging, you will need to be able to turn it off. In privileged EXEC mode,
enter the command undebug all to turn off all debugging.
With all these tools in hand, you should be able to verify and troubleshoot IGRP operation
accurately. With practice, you should be able to troubleshoot IGRP in no time. Now that you
have a full understanding of how IGRP functions, how to configure IGRP, and how to verify
and troubleshoot IGRP operation, let’s move on to the big brother of IGRP: EIGRP.