4.3 Configuring VLANs
4.3.6 Access links and trunk links
An access link is a link on the switch that is a member of only one VLAN. This VLAN is referred to as the native VLAN of the port. Any device that is attached to the port is completely unaware that a VLAN exists. The device simply assumes that it is part of a network or subnet based on the Layer 3 information that is configured on the device. In order to ensure that it does not have to understand that a VLAN exists, the switch is responsible for removing any VLAN information from the frame before it is sent to the end device. Because only one VLAN is configured on the port, other VLANs cannot communicate with the device unless the information is routed by a Layer 3 device.

A trunk link differs from an access link in that it is capable of supporting multiple VLANs. Trunk links are typically used to connect switches to other switches or routers. Switches support trunk links on both Fast Ethernet and Gigabit Ethernet ports.

The switch has two methods of identifying the VLAN that a frame belongs to when the switch receives the frame on a trunk link. The identification techniques currently used are the Cisco proprietary ISL standard and the IEEE 802.1Q standard.

It is important to understand that a trunk link does not belong to a specific VLAN. The responsibility of a trunk link is to act as a conduit for VLANs between switches and routers. The trunk link can be configured to transport all VLANs or to transport a limited number of VLANs.

A trunk link may, however, have a native VLAN. The native VLAN of the trunk is the VLAN that the trunk uses if the trunk link fails for any reason.

In the Figure, Port A and Port B have been defined as access links on the same VLAN. By definition, they can belong to only VLAN 200 and cannot receive frames with a VLAN identifier. As Switch Y receives traffic from Port A destined for Port B, Switch Y will not add an ISL encapsulation to the frame.

Port C is also an access link. As you can see, Port C has been defined as a member of VLAN 200. If Port A sends a frame destined for Port C, the switch does the following:

  1. Switch Y receives the frame and identifies it as traffic destined for VLAN 200 by the VLAN and port number association.
  2. Switch Y encapsulates the frame with an ISL header identifying VLAN 200 and sends the frame through the intermediate switch on a trunk link.
  3. This process is repeated for every switch that the frame must transit as it moves to its final destination of Port C.
  4. Switch Z receives the frame, removes the ISL header, and forwards the frame to Port C.