An artist's depiction of an Extended Unit Death
I found this huge page thing that you may or may not have seen already:
http://midnight.hushedcasket.com/2007/01/30/make-your-router-xbox-live-compatible/If you think it's an issue with your specific Cisco router, you should try the
Cisco Support Forums for an answer.
Having the 360 on a static IP and forwarding the right ports should be all there is to it. Wish I could help more.