1. Overview
    1. Tagged frames (Ethertype 0x8100) encapsulated within additional 4 byte 802.1q header (EtherType 0x88a8), so system mtu 1504 must be added to all switches
    2. Use the vlan dot1q tag native global command to configure the edge switch so that all packets going out IEEE 802.1q trunk, including the native VLAN, are tagged. VLAN1 is a default native VLAN, so by default this command is required.
    3. the native VLANs of the IEEE 802.1Q trunks must not match any native VLAN of the nontrunking (tunneling) port on the same switch
    4. Supports CDP, STP, MSTP, VTP, PAgP, LACP, and UDLD
  2. Packet Double Tag
    1. Add Tag to original 802.1q Tagged Frame
  3. Convention
    1. 1. The inner tag is the tag which is closest to the payload portion of the frame; it is officially called C-TAG (Customer tag, with ethertype 0x8100).
    2. 2. The outer tag is the one closer/closest to the Ethernet header; its name is S-TAG (Service tag, ethertype 0x88a8).
    3. 3. tag 1 is the outer tag; tag 2, the second tag, is the inner tag. The tag number has nothing to do with the order in which the tags were added, etc. It is simply a convention.
    4. 4. For a single-tagged (802.1q) frame, that tag is designated tag 1 when mixed with 802.1ad tags.
    5. 5. In frames having more than one tag, the tags are numbered 1 to N, and appear sequentially and contiguously in the frame from Ethernet header to payload. In this case the innermost tag is the C-TAG and all other tags are S-TAGs.
  4. Commands
    1. show system mtu
    2. (G)system mtu 1504
    3. (IF)switchport access vlan {{ vlan-id }}
    4. (IF)switchport mode dot1q-tunnel
    5. (IF)l2protocol-tunnel cdp
    6. (IF)l2protocol-tunnel stp
    7. (IF)l2protocol-tunnel vtp
    8. (IF)l2protocol-tunnel point-to-point pagp
    9. (IF)l2protocol-tunnel point-to-point lacp
    10. (IF)l2protocol-tunnel point-to-point udld
    11. show dot1q-tunnel