Configuring VLAN Trunks

Configuring a Trunk Port

This section describes how to use the CLI to configure an ISL or IEEE 802.1Q trunk port, how to define the VLANs that can use a port, and how to disable a trunk port.

If you are assigning a port on a cluster member switch to a VLAN, first log in to the member switch by using the privileged EXEC rcommand command. See the Cisco IOS Desktop Switching Command Reference for more information on how to use this command.

To define a port as an ISL trunk port, perform this task from privileged EXEC mode:

Task	Command
Step 1 Enter global configuration mode.	configure terminal
Step 2 Enter the interface configuration command mode and the port to be added to the VLAN.	interface interface_id
Step 3 Configure the port with a VLAN membership mode of trunk.	switchport mode trunk
Step 4 1 Configure the port to support ISL trunking.	switchport trunk encapsulation isl
Step 5 Return to privileged EXEC mode.	end
Step 6 Verify your entries.	show interface interface-id switchport
Step 7 Save the configuration.	copy running-config startup-config

This example shows how to configure a port as a trunk, verify the trunk configuration, and save the change to the startup configuration file:

Switch# configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Switch(config)# interface fa0/1

Switch(config-if)# switchport mode trunk

Switch(config-if)# switchport trunk encapsulation isl

Switch(config-if)# end

Switch# show interface fa0/1 switchport

Name: Fa0/1

Switchport: Enabled

Administrative mode: trunk

Operational Mode: trunk

Administrative Trunking Encapsulation: isl

Operational Trunking Encapsulation: isl

Negotiation of Trunking: Disabled

Access Mode VLAN: 0 ((Inactive))

Trunking Native Mode VLAN: 1 (default)

Trunking VLANs Enabled: 1-3,1002-1005

Trunking VLANs Active: 1-3

Pruning VLANs Enabled: NONE

Defining the Allowed VLANs on a Trunk

A trunk port by default sends to and receives traffic from all VLANs in the VLAN database. All VLANs, 1 to 1005, are allowed on each trunk. However, you can remove VLANs from the allowed list, preventing traffic from those VLANs from passing over the trunk. To restrict the traffic a trunk carries, use the remove vlan-list parameter to remove specific VLANs from the allowed list.

To modify the allowed list of a trunk, perform this task from privileged EXEC mode:

Task	Command
Step 1 Enter global configuration mode.	configure terminal
Step 2 Enter the interface configuration command mode and the port to be added to the VLAN.	interface interface_id
Step 3 Configure the VLAN membership mode for trunks.	switchport mode trunk
Step 4 Define the VLANs that are not allowed to transmit and receive on the port. The vlan-list parameter is a range of VLAN IDs separated by a hyphen or specific VLAN IDs separated by commas.	switchport trunk allowed vlan remove vlan-list
Step 5 Return to privileged EXEC.	end
Step 6 Verify your entries.	show interface interface-id switchport allowed-vlan
Step 7 Save the configuration.	copy running-config startup-config

This example shows how to define the allowed VLANs list for trunk port Fa0/1 to allow VLANs 1 to 100, VLAN 250, and VLANs 500 to 1005, and how to verify the allowed VLAN list for the trunk:

Switch(config)# interface fa0/1

Switch(config-if)# switchport mode trunk

Switch(config-if)# switchport trunk allowed vlan remove 101-499

Switch(config-if)# switchport trunk allowed vlan add 250

Switch(config-if)# end

Switch# show interface fa0/1 switchport allowed-vlan

"1-100,250,500-1005"

Switch#

Disabling a Trunk Port

You can disable trunking on a port by returning it to its default static-access mode. To disable trunking on a port, perform the following tasks from privileged EXEC mode:

Task	Command
Step 1 Enter global configuration mode.	configure terminal
Step 2 Enter the interface configuration command mode and the port to be added to the VLAN.	interface interface_id
Step 3 Return the port to its default static-access mode.	no switchport mode
Step 4 Return to privileged EXEC.	end
Step 5 Verify your entries.	show interface interface-id switchport

This example shows how to disable trunking on a port:

Switch# configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

Switch(config)# interface fa0/1

Switch(config-if)# no switchport mode

Switch(config-if)# end

Switch# show interface fa0/1 switchport

Name: Fa0/1

Switchport: Enabled

Administrative mode: static access

Operational Mode: static access

Administrative Trunking Encapsulation: isl

Operational Trunking Encapsulation: isl

Negotiation of Trunking: Disabled

Access Mode VLAN: 1 (default)

Trunking Native Mode VLAN: 1 (default)

Trunking VLANs Enabled: NONE

Pruning VLANs Enabled: NONE