Posted in Networking

Frame Tagging Facts

Although you can create VLANs with only one switch, most networks involve connecting multiple switches. The area between switches is called the switch fabric. As a frame moves from switch to switch within the switch fabric, each switch must be able to identify the destination virtual LAN.

One way to identify the VLAN is for the switch to use a filtering table that maps VLANs to MAC addresses. However, this solution does not scale well. For large networks, switches append a VLAN ID to each frame. This process, called frame tagging or frame coloring, identifies the VLAN of the destination device.

Remember the following facts regarding switch frame tagging (or coloring).

  • VLAN IDs identify the VLAN of the destination device.
  • Tags are appended by the first switch in the path, and removed by the last.
  • Only VLAN-capable devices understand the frame tag.
  • Tags must be removed before a frame is forwarded to a non-VLAN-capable device.
  • Tag formats and specifications can vary from vendor to vendor. When designing VLANs, you might need to stick with one switch vendor. Cisco’s proprietary protocol is called the Inter-Switch Link (ISL) protocol. Use 802.1q-capable switches to ensure a consistent tagging protocol.

 

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