- IEEE 802.1Q (dot1q) is a industry standards based implementation of carrying traffic for multiple VLANs over a single trunk link between two Ethernet switches.
- This means adding an identifier (tag) to frames to indicate what VLAN the frame belongs to.
- 802.1Q does not encapsulate the original frame. Instead, for Ethernet frames, it adds a 32-bit (4-byte) field between the source MAC address and the EtherType/Length fields of the original frame.
- TPID - Tag Protocol Identifier (16 bits): set to a value of 0x8100 to identify the frame as an IEEE 802.1Q tagged frame.
- Priority (3 bits): indicates the priority level (0 through 7) used for QoS.
- CFI - Canonical Format Indicator (1 bit): specifies if the MAC address is in noncanonical (1) or canonical (0) format.
- VID - VLAN Identifier (12 bits): uniquely identifies the VLAN which the frame belongs to.
- In short, the ONLY difference between a regular VLAN and the native VLAN is that frames from/to the native VLAN are carried untagged.
- As explained in dot1q section, VLANs are identified by adding a tag to the frame.
- What happens if a trunk port receives a frame without this tag? Which VLAN does the frame belong to? Answer: the native VLAN.
- If a switch receives an untagged frame, it associates that frame to the native VLAN.
- When a switch sends out a frame belonging to the native VLAN, it removes the tag.
- The native VLAN is configurable on a per port basis with the switchport trunk native vlan <vlan-id> command.
- The default native VLAN is VLAN 1.
ReferencesISL and IEEE 802.1Q Frame Format
VLAN Trunking Using IEEE 802.1Q
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