if your Cisco router is running a recent advip IOS version, it comes with a built-in, application-layer packet sniffing application called Network Based Application Recognition (NBAR). This was originally designed for QoS.
You can try this:
using command
ip nbar protocol-discovery. That will enable your packet-sniffing application.
command
show ip nbar protocol-discovery stats bit-rate top-n 10. You'll be presented with an output that looks like this:
It even recognizes common p2p applications, kazaa, bittorent, napster, and so on...