Spanning Tree Protocol: The Root Port

Once Spanning Tree has successfully elected a Root Switch, all of the other switches now have to determine what port is considered the ‘Root Port’:

  1. The Root sends Hello BPDUs every Hello timer interval, normally every 2 seconds. These Hello BPDU messages contain the RBID and SBID set to the Id of the Root switch.  RPC is set to 0 and  the SPID is set to the ID of the egress port.
  2. Each nonroot switch receiving a BPDU on any port adds the ports cost to the RPC value in the received BPDU, determining the RESULTING BPDU.  The switch then declares the port receiving the superior RESULTING BPDU as its Root Port.
  3. Hello BPDUs received on the Root Port of a non root switch are forwarded through the remaining designated ports after updating the RPC, SBID, SPID, and MessageAge fields.  Hellos received on other ports on a nonroot switch are processed but they are not forwarded.
  4. Switches do not forward Hellos out Root Ports and ports that are put into a blocking state.

After this process results in each nonroot switch choosing exactly one port as it’s root port

rootportdetermination

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