CCIE 400-101: Layer 2 Technologies - dot1q, Native VLAN


  • 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.

    Native VLAN

    • 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.


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