[c-nsp] IP routing on lan subinterfaces..... cisco 3550

[deepak goel]

hi everybody,

I am doing this on cisco 3550, the image file is 
"c3550-i5k2l2q3-mz.121-22.EA6.bin"

My base interface looks like this....
!
interface FastEthernet0/8
no switchport
no ip address
!

Now I want to configure ip address on a subinterface.I have done these 
steps....

Cisco3550# configure terminal
Cisco3550(config)# interface fastEthernet0/8.1
Cisco3550(config-subif)# ip address <ip address> <netmask>

Configuring IP routing on a LAN subinterface is only allowed if that
subinterface is already configured as part of an IEEE 802.10, IEEE 
802.1Q,ISL vLAN.

cisco3550#

I dont know how to make subinterface a part of ISL vlan (as mentioned in 
error message). There is one command  "encapsulation isl vlan-id" but i 
think that is only present on cisco routers but not on switch.

Can anybody help me in this regard

Thanks
Deepak