Basic Troubleshooting

Viewing the Routing Table


Determining the Gateway of Last Resort


NOTE: The ip default-network command is for use with the deprecated Cisco proprietary Interior Gateway Routing Protocol (IGRP). Although you can use it with Enhanced Interior Gateway Routing Protocol (EIGRP) or RIP, it is not recommended. Use the ip route 0.0.0.0 0.0.0.0 command instead.

Routers that use the ip default-network command must have either a specific route to that network or a 0.0.0.0 /0 default route.


Determining the Last Routing Update



OSI Layer 3 Testing



Interpreting the show interface Command


The traceroute Command



The show controllers Command


debug Commands


CAUTION: Turning all possible debugging on is extremely CPU intensive and will probably cause your router to crash. Use extreme caution if you try this on a production device. Instead, be selective about which debug commands you turn on.

Do not leave debugging turned on. After you have gathered the necessary information from debugging, turn all debugging off. If you want to turn off only one specific debug command and leave others on, issue the no debug x command, where x is the specific debug command you want to disable.


Using Time Stamps


TIP: Make sure you have the date and time set with the clock command at privileged mode so that the time stamps are more meaningful.


Operating System IP Verification Commands

The following are commands that you should use to verify what your IP settings are. Different operating systems have different commands.

• ipconfig (Windows 2000/XP):
Click Start > Run > Command > ipconfig or ipconfig/all.

• winipcfg (Windows 95/98/Me):
Click Start > Run > winipcfg.

• ifconfig (Mac/Linux):
#ifconfig


The ip http server Command


CAUTION: The HTTP server was introduced in Cisco IOS Software Release 11.0 to extend router management to the web. You have limited management capabilities to your router through a web browser if the ip http server command is turned on.

Do not turn on the ip http server command unless you plan to use the browser interface for the router. Having it on creates a potential security hole because another port is open.


The netstat Command