802.1Q IEEE Standard for VLAN Tagging
The industry-standard protocol that allows multiple Virtual LANs (VLANs) to multiplex and travel across a single physical trunk link between switches. Without 802.1Q, a switch receiving a frame on a trunk would have no idea which broadcast domain the traffic belonged to.
The Frame Injection Process
Unlike Cisco's legacy ISL protocol (which encapsulated the entire frame), 802.1Q performs an "internal injection." When a frame moves from an access port onto a trunk port, the switch literally slices the Ethernet frame open and inserts a 4-byte (32-bit) tag directly between the Source MAC Address and the EtherType field.
| Tag Component | Function & Size |
|---|---|
| TPID | Tag Protocol Identifier (16 bits): Always set to 0x8100. This hexadecimal value alerts the receiving switch that the frame has been tagged. |
| PCP / CoS | Priority Code Point (3 bits): Used for Layer 2 Quality of Service (QoS). It allows voice or video traffic to be prioritized over a congested trunk link. |
| VID | VLAN Identifier (12 bits): The actual VLAN number (1 through 4094). This dictates exactly which VLAN the frame gets dropped into on the receiving switch. |
CCNA Exam Gotchas
The Native VLAN Exception
By default, 802.1Q uses VLAN 1 as the Native VLAN. Any traffic assigned to the Native VLAN is explicitly sent across the trunk link without an 802.1Q tag. If a switch receives an untagged frame on a trunk port, it automatically forwards that frame into whatever its locally configured Native VLAN is. If the Native VLANs do not match on both sides of the link, traffic will leak between VLANs, causing a massive security vulnerability and a spanning-tree error.