Cisco Systems 3750E manual Defining the Allowed VLANs on a Trunk, 13-21

Chapter 13 Configuring VLANs

Configuring VLAN Trunks

	Command	Purpose
Step 4
	switchport mode {dynamic {auto	Configure the interface as a Layer 2 trunk (required only if the interface
	desirable} trunk}	is a Layer 2 access port or tunnel port or to specify the trunking mode).
		• dynamic auto—Set the interface to a trunk link if the neighboring
		interface is set to trunk or desirable mode. This is the default.
		• dynamic desirable—Set the interface to a trunk link if the
		neighboring interface is set to trunk, desirable, or auto mode.
		• trunk—Set the interface in permanent trunking mode and negotiate
		to convert the link to a trunk link even if the neighboring interface is
		not a trunk interface.
Step 5
	switchport access vlan vlan-id	(Optional) Specify the default VLAN, which is used if the interface stops
		trunking.
Step 6
	switchport trunk native vlan vlan-id	Specify the native VLAN for IEEE 802.1Q trunks.
Step 7
	end	Return to privileged EXEC mode.
Step 8
	show interfaces interface-idswitchport	Display the switchport configuration of the interface in the Administrative
		Mode and the Administrative Trunking Encapsulation fields of the
		display.
Step 9
	show interfaces interface-idtrunk	Display the trunk configuration of the interface.
Step 10
	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To return an interface to its default configuration, use the default interface interface-idinterface configuration command. To reset all trunking characteristics of a trunking interface to the defaults, use the no switchport trunk interface configuration command. To disable trunking, use the switchport mode access interface configuration command to configure the port as a static-access port.

This example shows how to configure a port as an IEEE 802.1Q trunk. The example assumes that the neighbor interface is configured to support IEEE 802.1Q trunking.

Switch# configure terminal

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

Switch(config)# interface gigabitethernet1/0/2

Switch(config-if)#switchport mode dynamic desirable

Switch(config-if)#switchport trunk encapsulation dot1q

Switch(config-if)# end

Defining the Allowed VLANs on a Trunk

By default, a trunk port sends traffic to and receives traffic from all VLANs. All VLAN IDs, 1 to 4094, 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 switchport trunk allowed vlan remove vlan-listinterface configuration command to remove specific VLANs from the allowed list.

Note VLAN 1 is the default VLAN on all trunk ports in all Cisco switches, and it has previously been a requirement that VLAN 1 always be enabled on every trunk link. You can use the VLAN 1 minimization feature to disable VLAN 1 on any individual VLAN trunk link so that no user traffic (including spanning-tree advertisements) is sent or received on VLAN 1.

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02		13-21