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VLAN Trunk

Edgar c Francis

A Trunk link is a point-to-point link between two network devices. Trunk link carry more than one VLAN. With VLAN trunking, we can extend our configured VLAN across the entire network. Remember, sending information from an access link on one VLAN to another VLAN is not possible without the additional device a router or a external layer 2 bridge connected between the VLAN.


A Trunk link can transport multiple VLANs traffic through a single switch port. A trunk link is not assigned to a specific VLAN.


Types of links/ports.

Access links

  • Connecting to end devices (hosts or router)
  • Part of one VLAN

Trunk links

  • Do not belong to any VLAN
  • Its Carry multiple VLANs traffic
  • It’s a physical Link between two switches

Frame tagging

In order to make sure that same VLAN users on different switch communicate with each other there is a method of tagging happens on trunk links. Tag is added before a frame is send and removed once it is received on trunk link. Frame tagging happen only on the trunk links.

VLAN Frame Identification


A Trunk link can transport multiple VLANs traffic; a switch must identify frames with their respective VLANs as they are sent and received over a trunk link.


VLAN identification can be performed using  two methods, each using  a different frame identification mechanism:

  • Inter-switch link (ISL) protocol
  • IEEE 802.1Q (Dot1Q) protocol


Inter-switch link (ISL) is a Cisco-proprietary method for preserving the source VLAN identification of frame passing over trunk link. ISL work with Ethernet, Token ring, FDDI. ISL adds a 26-bytes header and a byte trailer to the frame you can say its adds 30 bytes of tag. The ISL method of VLAN identification or trunking encapsulation no longer is supported across all Cisco catalyst switch platform.


The IEEE 802.1Q protocol is also used to carry VLAN traffic, dot1q is a open standard trunking protocol, its work only on Ethernet, dot1q introduce us the concept of Native VLAN on a trunk. The frame belong to native VLAN is not encapsulation and with any tag. Dot1q is only adding 4 bytes tag will be added to original frame just after the source address field.


Lets see the configuration:


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