There is a requirement is task 7.5 to run IPv6 in VLAN 46. However one of the switch (SW4) is 3550 which does not forward IPv6 traffic.
I know that we could use fallback switching on 3550 to allow IPV6 traffic. Unforunately I can't make it work. Could someone help me?

Here are the network connections digram and configurations:

|R4|fa0/1 -- fa0/4|SW4|fa0/21 -- fa0/21|SW3| fa0/16 -- fa0/16|SW2|fa0/6 -- fa0/0|R6|

R4
int fa0/1
ipv addr 2001:CC1E:16:46::4/64
!

R6
int fa0/0
ipv addr 2001:CC1E:16:46::6/64
!

SW2 (3560)
bridge 1 protocl vlan-base
bridge irb
bridge 1 route ip
int vlan 46
bridge-group 1
!
int fa0/6
no switchp
bridge-group 1
!

SW3 (3550)
bridge 1 protocol vlan-based
bridge 1 irb
bridge 1 route ip
int vlan 46
bridge-group 1
!

SW4 (3550)
bridge 1 protocol vlan-based
bridge 1 irb
bridge 1 route ip
int vlan 46
bridge-group 1
!
int fa0/4
no switchport
bridge-group 1
!