encapsulation on 2600 router

Hi,
I’m working on 2600 Cisco routers and trying to configure ISL encapsulation on a fast ethernet subinterface for inter-VLAN communication. So, I’m connecting a 2600 Cisco router and a 5000 Cisco switch. On the 2600s, which are running IOS versions 11.2 – 12.0, the required command (encaps isl [vlan #]) does not seem to be available. What can I do to assign ISL encapsulation and a VLAN to the subinterface?