Spanning Tree Protocol: Port Selection

Spanning Tree Protocol determines which ports are put into a forwarding or blocking state.  The combination of ports that are forwarding or blocking for each VLAN creates a loop free broadcast domain.  STP follows a three step process to determine which ports to forward and block:

  1. Elect the Root switch
    • The switch with the lowest bridge ID is elected the root switch
  2. Determine the Root Port
    • The single port on the switch that receives the superior BPDUs from all received BPDUs on all other ports
  3. Determine the Designated Port for each segment
    • When multiple ports are connected on the same segment, this port is the one that forwards the superior BPDU with least RPC onto that segment



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