If you run a port scan again your router, you'll typically see things like http, snmp, finger, chargen, and other services. You'll also see the ports for reverse telnet.
The following doc is a good starting point for securing your router:
Possibly Unnecessary Services
http://www.cisco.com/warp/public/707/21.html#possibly_unnecessary
Improving Security on Cisco Routers
http://www.cisco.com/warp/public/707/21.html
HTH
Jeff
