108 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 9 Copyright © 2004, Cisco Systems, Inc.
This is an important concept for the students to understand to help in the troubleshooting
process.
Two important facts for the students to learn are that CDP is Cisco proprietary, which means
that it only works between Cisco devices, and that CDP only works between directly
connected devices.
9.3.4 Troubleshooting using traceroute The traceroute command is used to discover the routes that packets take when they travel
to their destination. The traceroute command can also be used to test the network layer on
a hop-by-hop basis.
The traceroute command output shows a list of hops that the traceroute successfully
reaches. Traceroute output can also be used to indicate the specific hop at which the failure
occurs. Traceroute also provides output that indicates the relative performance of the links.
There must be routes in both directions for the traceroute or ping data to successfully
make a round trip between routers. This is an important concept for the students to understand
to help in the troubleshooting process. Help students associate traceroute with determining
actual path.
9.3.5 Troubleshooting routing issues The show ip route command is perhaps the most important command for troubleshooting
routing issues. This displays the contents of the IP routing table. The output shows the entries
for all known networks and subnetworks and how that information was learned.
If the output of the show ip route command does not show the expected learned routes,
then it is possible that routing information is not being exchanged. In this case, use the show
ip protocols command on the router to check for a possible misconfigured routing
protocol.
The show ip protocols command output can be used to diagnose a multitude of routing
issues. It can be used to identify a router that is the source of incorrect router information. This
is an important concept for the students to understand to help in the troubleshooting process.
In the output of the show ip route command, clarify the values that represent the
administrative distance, metric, next hop interface, and the update time. In the show
protocols output, clarify the values that represent the update timers, the networks being
routed, and the routing sources.
9.3.6 Troubleshooting using show controllers The show controller serial command can be used to identify the type of cable that is
connected to the routers without inspecting the cables. This can be used to find a serial
interface with no cable, the wrong type of cable, or a defective cable.
The show controller serial command queries the integrated circuit chip that controls
the serial interface and displays information about the physical interface. It also produces a
tremendous amount of output, which includes the cable type. Most of the output is internal
technical details about the controller chip status. This information requires specific knowledge
of the integrated circuit. This is an important concept for the students to understand to help in