Defining the Spanning Tree Protocol – STP
802.1D port roles

On a non-root bridge, as spanning tree receives BPDUs on various ports, it determines the roles that each port will fill in the topology. There are four 802.1D port roles.

  • Root port – This port exists on non-root bridges and is the switch port with the best path to the root bridge. Root ports forward traffic toward the root bridge, and the source MAC address of frames received on the root port are capable of populating the MAC table. Only one root port is allowed per bridge.
  • Designated port – This port exists on root and non-root bridges. For root bridges, all switch ports are designated ports. For non-root bridges, a designated port is the switch port that will receive and will forward frames toward the root bridge as needed. Only one designated port is allowed per segment. If multiple switch exist on the same segment, an election process determines the designated switch, and the corresponding switch port begins forwarding frames for the segment. Designated ports are capable of populating the MAC table.
  • Nondesignated port – This is a switch port not forwarding (blocking) data frames and not populating the MAC address table with SA of frames seen on that segment.
  • Disabled port – This is a switch port that is shut down.

By examining the switch port roles on a switch, the forwarding path can be determined to ensure that data frames are taking the desired path.