Verify VLANs and trunking.
Use the show ip interface trunk command on S1 and the show vlan command on S2 to determine that the switches are trunking correctly and the proper VLANs exist.
S1#show interface trunk
Port Mode Encapsulation Status Native vlan
Fa0/1 on 802.1q trunking 1
Fa0/2 on 802.1q trunking 1
Fa0/3 on 802.1q trunking 1
Fa0/4 on 802.1q trunking 1
Fa0/5 on 802.1q trunking 1
Port Vlans allowed on trunk
Fa0/1 1-4094
Fa0/2 1-4094
Fa0/3 1-4094
Fa0/4 1-4094
Fa0/5 1-4094
Port Vlans allowed and active in management domain
Fa0/1 1,10,20,88
Fa0/2 1,10,20,88
Fa0/3 1,10,20,88
Fa0/4 1,10,20,88
Fa0/5 1,10,20,88
Port Vlans in spanning tree forwarding state and not pruned
Port Vlans in spanning tree forwarding state and not pruned
Fa0/1 1,10,20,88
Fa0/2 none A ← blocked due to spanning tree
Fa0/3 1,10,20,88
Fa0/4 1,10,20,88
Fa0/5 1,10,20,88>