We have a Cisco 2621 running IOS Software (C2600-IO3-M), Version 12.3(9).
I can't seem to figure out how to enable 802.1q encapsulation on the subinterfaces. All the documentation I can find on Cisco's site refers to version 12.1 using the enscapsulation command (easy 'nuff). There is no sign of encap in any of the global config commands.
Is the VLAN feature set supported in this IOS (IO3-M) version? If so, what the heck are the encapsulation commands?