VLAN frame encapsulation

To identify Ethernet frames from different VLANs, IEEE 802.1Q inserts a four-byte VLAN tag between the destination and source MAC address (DA&SA) field and the Type field.

A VLAN tag includes the following fields:

• TPID—16-bit tag protocol identifier that indicates whether a frame is VLAN-tagged. By default, the TPID value 0x8100 identifies a VLAN-tagged frame. A device vendor can set the TPID to a different value. For compatibility with a neighbor device, set the TPID value on the device to be the same as the neighbor device.

• Priority—3-bit long, identifies the 802.1p priority of the frame. For more information, see ACL and QoS Configuration Guide.

• CFI—1-bit long canonical format indicator that indicates whether the MAC addresses are encapsulated in the standard format when packets are transmitted across different media. Available values include:

  • 0 (default)—The MAC addresses are encapsulated in the standard format.

  • 1—The MAC addresses are encapsulated in a non-standard format.

  This field is always set to 0 for Ethernet.

• VLAN ID—12-bit long, identifies the VLAN to which the frame belongs. The VLAN ID range is 0 to 4095. VLAN IDs 0 and 4095 are reserved, and VLAN IDs 1 to 4094 are user configurable.

The way a network device handles an incoming frame depends on whether the frame has a VLAN tag and the value of the VLAN tag (if any). For more information, see "Introduction."

Ethernet supports encapsulation formats Ethernet II, 802.3/802.2 LLC, 802.3/802.2 SNAP, and 802.3 raw. The Ethernet II encapsulation format is used here. For information about the VLAN tag fields in other frame encapsulation formats, see related protocols and standards.

For a frame that has multiple VLAN tags, the device handles it according to its outermost VLAN tag and transmits its inner VLAN tags as the payload.