IEEE’s 802.1Q VLAN tagging is the industry standard. Inter-Switch Link (ISL) is a Cisco proprietary VLAN tagging protocol and if you have networking devices from different vendors, IEEE’s 802.1Q VLAN tagging must be used for interoperability.
Inter-Switch Link (ISL) encapsulates an Ethernet Frame with ISL header and FCS but IEEE 802.1Q uses an internal tagging process by inserting the VLAN tagging information inside Ethernet Frame. This method of tagging is known as internal tagging.
802.1Q trunks support tagged and untagged frames. An untagged frame is a standard unaltered Ethernet frame. Untagged frames are usually used for native VLAN communication. If a switch receives untagged frames on a trunk port, they are considered as part of the native VLAN and frames from a native VLAN access port are not tagged when exiting the switch via a native VLAN trunk port.
In a tagged 802.1Q frame, a 4-byte field is inserted between the original Ethernet frame Source Address field and the Type or Length field. FCS is recomputed after the 4-byte tag is inserted. Following figure shows 802.1Q tagged Ethernet frame.
Following are the fields in an 802.1Q VLAN tag.
• TPID (Tag Protocol Identifier, 16 bits): TPID (Tag Protocol Identifier) is globally and always have a value of 0x8100 to signify an 802.1Q tag.
• Priority (3 bits): The Priority field is used by 802.1Q to implement Layer 2 quality of service (QoS).
• CFI (Canonical Format Identifier, 1 bit): The CFI (Canonical Format Identifier) bit is used for compatibility purposes between Ethernet and Token Ring.
• VLAN ID (12 bits): The VID field is used to distinguish between VLANs on the link.