Configure sub-interfaces on a switch port to support multiple VLANs on a single physical interface, commonly used for router-on-a-stick configurations.
Step 1: Enter the physical interface
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SW1>enable
SW1#configure terminal
SW1(config)#interface gigabitEthernet 0/1
SW1(config-if)#description Trunk to Router
SW1(config-if)#switchport mode trunk
SW1(config-if)#switchport trunk allowed vlan 10,20,30
SW1(config-if)#no shutdown
SW1(config-if)#exit
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Step 2: Create sub-interfaces for each VLAN
Configure sub-interfaces on the router side to handle inter-VLAN routing. Each sub-interface corresponds to a VLAN.
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R1>enable
R1#configure terminal
R1(config)#interface gigabitEthernet 0/0.10
R1(config-subif)#description VLAN 10 Gateway
R1(config-subif)#encapsulation dot1Q 10
R1(config-subif)#ip address 192.168.10.1 255.255.255.0
R1(config-subif)#no shutdown
R1(config-subif)#exit
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Step 3: Configure additional sub-interfaces
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R1(config)#interface gigabitEthernet 0/0.20
R1(config-subif)#description VLAN 20 Gateway
R1(config-subif)#encapsulation dot1Q 20
R1(config-subif)#ip address 192.168.20.1 255.255.255.0
R1(config-subif)#no shutdown
R1(config-subif)#exit
R1(config)#interface gigabitEthernet 0/0.30
R1(config-subif)#description VLAN 30 Gateway
R1(config-subif)#encapsulation dot1Q 30
R1(config-subif)#ip address 192.168.30.1 255.255.255.0
R1(config-subif)#no shutdown
R1(config-subif)#exit
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Step 4: Enable the parent interface
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R1(config)#interface gigabitEthernet 0/0
R1(config-if)#description Trunk from Switch
R1(config-if)#no shutdown
R1(config-if)#end
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Step 5: Verify the configuration
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R1#show ip interface brief
R1#show interfaces gigabitEthernet 0/0.10
SW1#show interfaces trunk
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Key Points:
- Sub-interface number doesn’t have to match VLAN ID, but it’s best practice
- Parent interface must be up for sub-interfaces to function
- Each sub-interface needs
encapsulation dot1Q command with correct VLAN tag
- Switch port must be configured as trunk with allowed VLANs