To configure an interface to be an access interface, the switchport mode acess interface command is used. This type of interface can be assigned only to a single VLAN.
To configure a trunk interface, the switchport mode trunk interface command is used. This type of interface can carry traffic of multiple VLANs.
An example will help you understand the concept.
Host A and host B are in different VLANs, VLAN 1 and VLAN 2. These ports need to be configured as access ports and assigned to their respective VLANs by using the following sequence of commands:
SW1(config-if)# SW1(config-if)#interface fastEthernet 0/1 SW1(config-if)#switchport mode access SW1(config-if)#exit SW1(config)#vlan 2 SW1(config-vlan)#interface fastEthernet 0/2 SW1(config-if)#switchport mode access SW1(config-if)#switchport access vlan 2 SW1(config-if)#
Because the link between SW1 and SW2 needs to carry traffic of multiple VLANs, it needs to be configured as a trunk interface. This is done by using the following commands on both SW1 and SW2:
SW1(config)#interface fastEthernet 0/3 SW1(config-if)#switchport mode trunk
SW2(config)#interface fastEthernet 0/1 SW2(config-if)#switchport mode trunk
Now the link between SW1 and SW2 can carry traffic from both the VLAN1 and VLAN2. You can verify that an interface is indeed a trunk interface by using the show interface Fa0/3 switchport command on SW1:
SW1#show interfaces fastEthernet 0/3 switchport Name: Fa0/3 Switchport: Enabled Administrative Mode: dynamic auto Operational Mode: down Administrative Trunking Encapsulation: dot1q Operational Trunking Encapsulation: native Negotiation of Trunking: On Access Mode VLAN: 1 (default) Trunking Native Mode VLAN: 1 (default) Voice VLAN: none Administrative private-vlan host-association: none Administrative private-vlan mapping: none Administrative private-vlan trunk native VLAN: none Administrative private-vlan trunk encapsulation: dot1q Administrative private-vlan trunk normal VLANs: none Administrative private-vlan trunk private VLANs: none Operational private-vlan: none Trunking VLANs Enabled: All Pruning VLANs Enabled: 2-1001 Capture Mode Disabled Capture VLANs Allowed: ALL Protected: false Unknown unicast blocked: disabled Unknown multicast blocked: disabled Appliance trust: none SW1#
VLAN 1 doesn’t have to be created, it exists by default. Also, by default, all ports are in the VLAN 1, so Fa0/1 doesn’t need the switchport access vlan 1 command.
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