Information Technology Reference
In-Depth Information
Although it may also be possible to trace the path between source and destination using ping and the IP
record route option, traceroute is preferred because the record route option can alter the way in which
packets are forwarded by routers in the network, yielding incorrect path information.
Packet Debugging
The following example is applicable to Cisco 16xx, 25xx, 26xx, 36xx, 40xx, 45xx, 70xx, and 75xx series
routers. Consult the Cisco TAC web page www.cisco.com/tac for instructions on using this command on
other Cisco router platforms.
Caution
The
debug ip packet
command should always be run with an access list to restrict the
traffic that it will display. Failure to do so will almost certainly lock up the router. Even
with an access list, there is always a possibility that packet debugging will lock up the
router.
Do not
run this utility on a router that cannot be brought down or physically reset.
To use the
debug ip packet
command, you must do the following:
1.
Enter enable mode.
CiscoRtr>
enable
CiscoRtr#
2.
Disable console logging. (This command can be run with console logging, but this increases the
likelihood of locking up the router.)
CiscoRtr#
configure terminal
CiscoRtr(config)#
no logging console
CiscoRtr(config)#
end
CiscoRtr#
3.
Enable buffer logging.
CiscoRtr#
configure terminal
CiscoRtr(config)#
logging buffered
CiscoRtr(config)#
end
CiscoRtr#
4.
Turn on logging timestamps with millisecond output.
CiscoRtr#
configure terminal
CiscoRtr(config)#
service timestamps log datetime msec
CiscoRtr(config)#
end
CiscoRtr#
5.
Ensure that the router's CPU is not overloaded. This depends greatly on the amount of traffic to
capture and the type of router. As a general rule, a CPU percentage (highlighted here) less than 30
percent should be safe.
Note: This may still cause your router to lock up!
CiscoRtr#
show processes cpu
CPU utilization for five seconds:
2%
/0%; one minute: 0%; five minutes: 0%
PID Runtime(ms) Invoked uSecs 5Sec 1Min 5Min TTY Process
1 2075 308386 6 0.00% 0.00% 0.00% 0 Load Meter
2 273 55 4963 1.55% 0.18% 0.06% 0 Exec
[snip]
CiscoRtr#
