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New Member

Debug traffic ???

Can I use debug command to debug a fraffic of a port on cisco 3620 ?

sach as 80 WWW

If can ,please let me know ...thanks a lot...


Re: Debug traffic ???

If your purpose to debug the traffic is for troubleshooting ONLY,

you can try using an extended access-list with debug.

Using the debug, you may need to disable fast-switching

in an interface. The following config should only be used temporarily

because it will only permit TCP port 80 coming IN or going OUT the

router (depending on your implementation).

All other traffic will be DENIED.

access-list 100 permit tcp any any eq www

interface XXX

ip access-group 100 in (or out)

router#debug ip packet 100

Debugging ip packets may generate so much information

that may render the router unusable. Handle debugs with

great care.

Hope this helps.

New Member

Re: Debug traffic ???

Thank you for your answer

but I want to know why disable fast-switching? And not only for Troubleshooting only but also for security reason...

thanks again...

Re: Debug traffic ???

The access-list and access-group in the reply above will only allow TCP port 80 traffic in that interface. *All* other traffic will be dropped.

If you want to see whats coming on port 80 you could apply a similiar ACL with the log-input option to log it to the log for a few minutes.


access-list 101 permit tcp any any eq 80 log-input

access-list 101 permit ip any any


ip access-group 101 in

You could also debug the traffic without applying the Access-list to the interface.

to do this do:

under config: access-list 101 permit tcp any any eq 80

term mon

u all

debug interface s0/0

debug ip packet 101

this narrows the debugs to the s0/0 interface and only for tcp port 80 traffic

Type 'u all' to end the debug. Be ready to enter this, or hit up arrow 3 times to get the u all entered before the debug commands just incase there is a flood of traffic that overwhelms the console/telnet output and possibly the router. After doing a u all the debug may scroll by for awhile because it's buffered up...

The reason you want to disable fast-switching is because the first packet gets process-switched by CPU and then a cache entry is made and rest of flow is fast-switched. debug traffic only shows process-switched traffic.

New Member

Re: Debug traffic ???

Be very careful with the debug command, it may make your router unresponsive and require a reboot to clear. I would recommend using the logging function of your acl. Also, you may want to point your logging to a server/client to make sure you get everything. Otherwise, you may only get a couple minutes with the router's log.


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