switchport mode trunk

Switchport mode trunk

The switchport mode command allows us to configure the trunking operational mode on a Layer 2 interface on a Cisco IOS device. By entering the command switchport mode trunk we configure the interface to operate in trunk mode. A Layer 2 interface operating in trunk mode carries traffic for multiple VLANs.

Frames egressing (exiting) a trunk mode interface are tagged with an 802.1Q tag specifying the VLAN to which the frame belongs. For a trunk interface, one VLAN may be specified as the native VLAN. Outoging frames belonging to the native VLAN are not tagged with a VLAN-ID tag.

Let's take a look at an example:

The context sensitive help shows us that the options for the switchport mode command include:

• trunk
• dynamic
• trunk

We can use this command to set the interface to operate in trunk mode as follows:

The switchport mode trunk command may be followed by the commands:

• switchport trunk allowed vlan
• switchport trunk native vlan

By default, a trunk interface will carry traffic for all the VLANs defined on a switch. The switchport trunk allowed vlan command can be used to customize the list of VLANs that a trunk interface belongs to.

By default, the native (untagged) VLAN for a trunk interface is the default interface (VLAN 1). The switchport trunk native vlan command can be used to set the native VLAN for a trunk interface to a VLAN other than the default VLAN.

VLANs, trunking, VLAN tagging and inter-vlan routing are covered comprehensively in the following interactive course: Course 5 - VLANs & Trunks. The course features visual, animated explanations and hands-on interactive labs.