Unless something has changed, from what I understand, the 1720 will never support VLANs at all as the ethernet controller is not capable of doing so.
I actually am looking for direction on a similar topic. I'm looking for a VLAN capable IOS for the 1750. The only 1700 series router that supports VLANs today, as far as I know, is the 1751. Anyone know of a 1750 IOS with VLAN support?
Martin Weddle
ADP Dealer Services