Tagged and Untagged Ports

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Untagged Ports

• If a switch port will only ever participate in a single VLAN,
  • it can be configured as untagged
  • referred to as an access port or host port
  • uses the following port tagging logic:
    • If a frame is addressed to a port in the same VLAN on the same switch,
      • no tag needs to be added to the frame
    • If the frame needs to be transported over a trunk link,
      • the switch adds the relevant 802.1Q tag to identify the VLAN
      • then forwards the frame over the trunk port
    • If the switch receives an 802.1Q tagged frame on an access (untagged) port, it strips the tag before forwarding it to the host

Tagged Port

• A tagged port will normally be one that is operating as a trunk
  • capable of transporting traffic addressed to multiple VLANs using 802.1Q frame format
  • trunk might be used to connect switches or to connect a switch to a router
  • in some circumstances, a host attached to a port might need to be configured to use multiple VLANs and would need to be attached to a trunk port
    • E.g., virtualization host with multiple guest operating systems
      • virtual servers might need to be configured to use different VLANs