You'll need an FastEthernet Network Module and probably Enterprise feature set software. The router may only support ISL encapsulation as well...just make sure your switch is set to ISL. Here's an example from a Gigabit router anyway...hope it helps. Disregard the IRB.
interface Port-channel1
description ### Trunk Channel Interface ###
no ip address
no ip redirects
no ip directed-broadcast
no ip route-cache cef
no ip route-cache
no ip mroute-cache
hold-queue 300 in
!
interface Port-channel1.1
description ### VLAN 1 Virtual Router Interface ###
encapsulation dot1Q 1 native
no ip redirects
no ip directed-broadcast
no ip route-cache
no ip mroute-cache
bridge-group 1
!
!
interface Port-channel1.2
description ### VLAN 2 Virtual Router Interface ###
encapsulation dot1Q 2
ip address 172.16.2.3 255.255.255.0
ip helper-address 192.168.1.63
no ip redirects
no ip directed-broadcast
no ip route-cache
no ip mroute-cache
standby 2 priority 100 preempt
standby 2 ip 172.16.2.1
!
!
interface GigabitEthernet3
description *** Virtual Trunk Interface ***
no ip address
no ip directed-broadcast
no negotiation auto
channel-group 1
!
interface GigabitEthernet4
description *** Virtual Trunk Interface ***
no ip address
no ip directed-broadcast
no negotiation auto
channel-group 1
!
!
interface BVI1
ip address 192.168.1.2 255.255.255.0
no ip redirects
no ip directed-broadcast
ipx network 111
standby 1 priority 110 preempt
standby 1 ip 192.168.1.1
!
!
bridge 1 protocol ieee
bridge 1 route ip
bridge 1 route ipx
