Nov 21, 2011

Abstract of CCNA study guide-21 - Checking Connectivity and Troubleshooting

Checking Network Connectivity and Troubleshooting
You can use the ping and traceroute commands to test connectivity to remote devices, and both of them can be used with many protocols, not just IP. But don’t forget that the show ip route command is a good troubleshooting command for verifying your routing table and the show interfaces command will show you the status of each interface.
Debug is a troubleshooting command that’s available from the privileged exec mode of Cisco IOS. It’s used to display information about various router operations and the related traffic generated or received by the router, plus any error messages.
you really need to understand some important facts about its use:
 Debug is regarded as a very high-priority task because it can consume a huge amount of resources and the router is forced to process-switch the packets being debugged. So you don’t just use debug as a monitoring tool—it’s meant to be used for a short period of time and only as a troubleshooting tool.

it’s best to use more-specific debug commands.
As you can see from the following output, you can’t enable debugging from user mode, only
privileged mode:
Corp>debug ?
% Unrecognized command
Corp#debug all
This may severely impact network performance. Continue? (yes/[no]):yes
All possible debugging has been turned on
2d20h: SNMP: HC Timer 824AE5CC fired
2d20h: Rudpv1 Sent: Pkts 0, Data Bytes 0, Data Pkts 0
2d20h: Rudpv1 Discarded: 0, Retransmitted 0
To disable debugging on a router, just use the command no in front of the debug command:
Corp#no debug all
But I typically just use the undebug all command since it is so easy when using the shortcut:
Corp#un all
Remember that instead of using the debug all command, it’s almost always better to use specific commands—and only for short periods of time.
Corp#debug ip rip
RIP protocol debugging is on
1w4d: RIP: sending v2 update to via Serial0/0 (
1w4d: RIP: build update entries
1w4d: via, metric 2, tag 0
Corp#un all

Using the show processes Command
 the show processes (or show processes cpu) is a good tool for determining a given router’s CPU utilization. Plus, it’ll give you a list of active processes along with their corresponding process ID, priority, scheduler test (status), CPU time used, number of times invoked, and so on.
this command is super handy when you want to evaluate your router’s performance and CPU utilization.
Corp#sh processes
CPU utilization for five seconds: 2%/0%; one minute: 0%; five minutes: 0%
PID QTy PC Runtime (ms) Invoked uSecs Stacks TTY Process
1 Cwe 8034470C 0 1 0 5804/6000 0 Chunk Manager
6 Cwe 80355E5C 20 3 6666 5704/6000 0 Pool Manager
 [output cut]
So basically, the output from the show processes command shows that our router is happily able to process debugging commands without being overloaded.

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