IEEE 802.1Q

Jul 08,2008 00:00 by admin

IEEE 802.1Q

While ISL encapsulates an Ethernet frame with a 30-byte header, IEEE 802.1Q simply adds an additional 4-byte Tag field to the Ethernet frame (EtherType 0x8100). The Tag field has three components in addition to the EtherType:

Priority (3 bits)— The Priority field is used by 802.1p to implement Layer 2 quality of service (QoS).
Canonical Format Identifier (CFI) (1 bit)— The CFI bit is used for compatibility purposes between Ethernet and Token Ring.
VLAN ID (VID) (12 bits)— The VID field is used to distinguish between VLANs on the link.

FCS is recomputed after the 4-byte tag is inserted. IEEE 802.1Q supports up to 4096 VLANs because of the 12-bit length. The IEEE 802.1Q tag is not inserted on the native VLAN, which is the VLAN that the port was assigned to before becoming a trunk port. Figure 4-6 illustrates the IEEE 802.1Q tag format.