By default all VLANs configured on a switch are allowed over all trunking interfaces. Sometimes you need to change the allowed list over a specific trunk. When a VLAN is allowed on a trunk, traffic coming from interfaces belonging to this VLAN is allowed to traverse the trunk.
In our example we have R1 and R2 connected to two different switches. Both belong to VLAN 12 and the switches are connected by a dot1q trunk as shown in the diagram below.
Topology Configuration:
R1(config)#int f0/0 R1(config-if)#ip address 192.168.12.1 255.255.255.0 !-- R2 interface configuration R2(config)#int f0/0 R2(config-if)#ip address 192.168.12.2 255.255.255.0 !-- Sw1 Configuration SW1(config)#int f0/1 SW1(config-if)#description connected to R1 SW1(config-if)#switchport mode access SW1(config-if)#switchport access vlan 12 SW1(config-if)#int f0/13 SW1(config-if)#description connected to SW2 SW1(config-if)#switchport trunk encapsulation dot1q SW1(config-if)#switchport mod trunk !-- SW2 configuration SW2(config)#int f0/2 SW2(config-if)#description connected to R2 SW2(config-if)#switchport mod access SW2(config-if)#switchport access vlan 12 SW2(config-if)#int f0/13 SW2(config-if)#description connected to SW1 SW2(config-if)#switchport trunk encapsulation dot1q SW2(config-if)#switchport mode trunk
All configured VLANs are allowed on the trunk interface by default as shown in the output below:
SW1(config)#do sh int trunk Port Mode Encapsulation Status Native vlan Fa0/13 on 802.1q trunking 1 Port Vlans allowed on trunk Fa0/13 1-4094 Port Vlans allowed and active in management domain Fa0/13 1,3,5-6,8,10,12,26,33,52,100,255,783 Port Vlans in spanning tree forwarding state and not pruned Fa0/13 3,5-6,8,10,12,26,33,52,100,255,783 !-- SW2 trunk interface SW2(config)#do sh int trunk Port Mode Encapsulation Status Native vlan Fa0/13 on 802.1q trunking 1 Port Vlans allowed on trunk Fa0/13 1-4094 Port Vlans allowed and active in management domain Fa0/13 1,3,5-6,8,10,12,26,33,52,100,255,783 Port Vlans in spanning tree forwarding state and not pruned Fa0/13 1,3,5-6,8,10,12,26,33,52,100,255,783
R1 and R2 should be able to communicate as shown below:
R1#ping 192.168.12.2 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.12.2, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
Now I am going to remove VLAN 12 from the allowed list on the trunk:
SW1(config)#int f0/13 SW1(config-if)#switchport trunk allowed vlan remove 12 !-- VLAN 12 is removed from the allowed list SW1#sh interface trunk Port Mode Encapsulation Status Native vlan Fa0/13 on 802.1q trunking 1 Port Vlans allowed on trunk Fa0/13 1-11,13-4094 Port Vlans allowed and active in management domain Fa0/13 1,3,5-6,8,10,26,33,52,100,255,783 Port Vlans in spanning tree forwarding state and not pruned Fa0/13 3,5-6,8,10,26,33,52,100,255,783
R1 and R2 is not able to communicated any more over the trunk interface because frames tagged with VLAN 12 ID are not allowed to traverse the trunk anymore:
R1#ping 192.168.12.2 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.12.2, timeout is 2 seconds: ..... Success rate is 0 percent (0/5)
for more information on using the command please visit the command reference for show trunk
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