said by aryoba:
How did you mean by work?
For one, your 1721 router only has one 10/100 Ethernet port. Unless you add WIC-4ESW; how would you connect the router to the modem and the switch?
use a switch in front of your router with tagged-subif's.
disable stp, cdp on the modem-facing vlan so it doesn't think that your switch is the 'learned' mac address.
i used to do this when i had (3) static ips for my internet. i would use an "outside" vlan that i could plug in to with a workstation and get an outside ip address for testing vpn stuffs, etc.
"...if I in my north room dance naked, grotesquely before my mirror waving my shirt round my head and singing softly to myself..."