Spanning Tree Protocol: The Designated Port

In order for a switch to become a designated switch it must be able to forward the superior BPDUs from the root switch to other segments of the LAN.  The ports by which it forwards the superior BPDUs to other segments is called the Designated Port.  All other ports not designated as a root port or designated port are moved to a blocking state and are referred to as non-designated ports.

If there are multiple ports on a switch forwarding superior BPDUs to the same segment the following tie-breakers are:

  • The switch with the Least Cost Path to the root is selected, identified by the lowest Bridge ID.
  • The the neighboring switch with the lowest Bridge ID
  • Then finally the port on the neighbor with the lowest bridge ID with the lowest Port ID.

designatedport.PNG

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