Debug ip packet command
From NetworkCommands
Output displays IP addresses before AND after NATting.
Fast switching must be disabled on relevant interfaces as only process switched packets are displayed in debug output. Use the no ip route-cache command where necessary.
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Usage Examples
It's good practise to enter the undebug all command before you enter your actual debug command. This way, if you've made a mistake and/or the output is far greater than expected, you can simply press the up arrow once to bring up the last command and press enter to execute it and stop all debugging.
debug ip packet - Never a good idea! Display details of every ip packet passing through the router. Likely to crash the device unless very little traffic is being processed or it's extremely powerful.
debug ip packet 199 - A much better idea. Display details of only those packets matching the permit statements in the specified access list, 199 in this case.
debug ip packet 199 detail - Also displays source and destination ports and TCP flags such as SYN, ACK etc.
debug ip packet 199 dump - A recent addition. Also displays packet contents! Like a straight debug ip packet use this with great caution as the output is likely to be overwhelming.
Usage Notes
Disabling IP CEF will NOT have the same affect as using the no ip route-cache command.
You cannot use named access lists.
The access list should reference post-NAT addresses if NATting is occurring.
Additional Resources
This access list prevents most unwanted output;
access-list 199 deny udp any any eq rip access-list 199 deny udp any any eq 1984 access-list 199 deny udp any any eq 1985 access-list 199 deny udp any any eq 161 !SNMP access-list 199 deny udp any any eq 162 !SNMP Trap access-list 199 deny udp any any eq 49 !TACACS access-list 199 deny tcp any any eq 49 !TACACS access-list 199 deny udp any any eq 123 !NTP access-list 199 deny tcp any any eq 22 !SSH access-list 199 deny tcp any any eq 23 !TELNET access-list 199 deny ip any 224.0.0.0 255.255.255.0 !Multicast traffic; HSRP, OSPF etc. access-list 199 deny ip any host x.x.x.x !SNMP Server access-list 199 deny ip host x.x.x.x any !This device to anywhere first interface access-list 199 deny ip host x.x.x.x any !This device to anywhere second interface access-list 199 deny ip any host x.x.x.x !Anywhere to this device first interface access-list 199 deny ip any host x.x.x.x !Anywhere to this device second interface access-list 199 permit ip any any access-list 199 deny icmp any any !PING etc.
Related Commands
Use the undebug all command to stop all debugging.
