Trunking and IEEE 802.1Q

---

• a single switch will not provide enough ports for all the hosts that need to be connected to the network
  • means that multiple switches must be interconnected to build the network fabric
  • multiple switches may need to be deployed to provide redundant links
  • interconnections between switches are referred to as trunks
  • one port on each switch would be configured as the trunk port

• when frames designated for different VLANs are transported across a trunk,
  • the VLAN ID (VID) of each frame must be preserved for the receiving switch to forward it correctly
  • VIDs are defined by the IEEE 802.1Q standard
    • per-VLAN traffic is identified by a tag inserted in the Ethernet frame between the source address and EtherType fields
    • tag contains information about the VID and priority (for QoS functions)
    • the EtherType value is set to identify the frame as 802.1Q