Cisco IOS and Sniffing

How can I leverage Cisco IOS to sniff traffic?

I can span a port and connect a sniffer.

What else? NetFlow? ACLs? have lost track of all the tools in IOS that one can use to sniff traffic.

I want to see if the 7609 router is receiving aaa traffic (UDP 1812 and 1813) and vise versa.

Thanks