Having the ability to conduct packet captures is a valuable tool for troubleshooting connectivity issues within a network. IOS routers 12.4(20)T and up has the Embedded Packet Capture (EPC) built in to it.

How to do packet captures on a Cisco ASA

How to do packet capture on Cisco Nexus OS

Routers

(All commands can be done from the exec mode except for creating the ACL which requires config mode)

Define the capture point/interface/direction

monitor capture point ip cef CAPTURE FastEthernet0 both

Define the buffer and size and how to react when it fills up

monitor capture buffer CAPBUF size 512 max-size 1024 circular

Create an ACL to hit on the interesting traffic and assign it to the capture buffer

monitor capture buffer CAPBUF filter access-list ACL-CAP

Associate the capture point with the capture buffer

monitor capture point associate CAPTURE CAPBUF

Start the packet capture

monitor capture point start CAPTURE

To stop it:

monitor capture point stop CAPTURE

Now show what you’ve collected:

show monitor capture buffer CAPBUF [dump]

It is probably easier to read this in wireshark so here is how to export it:

monitor buffer export CAPBUF tftp://192.168.1.21/myrouter.pcap