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In order for two switches to participate in a Spanning Tree instance for a VLAN, they must each have the VLAN configured and the VLAN must be allowed on the trunk. For example, to have a Spanning Tree instance for VLAN 100 between SW1 and SW2, both need VLAN 100 configured:

SW1#conf t Enter configuration commands, one per line. End with CNTL/Z. SW1(config)#vlan 100 SW1(config-vlan)#no shut %VLAN 100 is not shutdown. SW1(config-vlan)#end SW1#show vlan id 100 VLAN Name Status Ports ---- -------------------------------- --------- ------------------------------- 100 VLAN0100 active Gi0/0, Gi0/1, Gi1/0, Gi1/1 Gi2/0, Gi2/1

VLAN 100 is active on the switch, meaning it's not shut down. Notice that it's active on six ports, all of which are terminating 802.1Q trunks. Using Gi0/0 as an example, we can see that VLAN 100 is allowed and active on the trunk:

SW1#show interfaces gi0/0 trunk Port Mode Encapsulation Status Native vlan Gi0/0 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/0 1-4094 Port Vlans allowed and active in management domain Gi0/0 1-10,100 Port Vlans in spanning tree forwarding state and not pruned Gi0/0 1-10,100

A VLAN must be configured on the switch before it can be allowed or active on a trunk. All VLANs are allowed on an 802.1Q trunk by default, but it is possible to remove or prune a VLAN, like so:

SW1(config)#int gi0/0 SW1(config-if)#switchport trunk allowed vlan remove 100 SW1(config-if)#do show interfaces gi0/0 trunk Port Mode Encapsulation Status Native vlan Gi0/0 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/0 1-99,101-4094 Port Vlans allowed and active in management domain Gi0/0 1-10 Port Vlans in spanning tree forwarding state and not pruned Gi0/0 1-10

To allow a VLAN on a trunk, replace remove in the preceding command with add:

SW1(config-if)#switchport trunk allowed vlan add 100

Be careful with this command. If you leave out add, then the trunk will allow only VLAN 100 and prune all other VLANs.