Implementing the Rapid Spanning Tree Protocol – RSTP
RSTP port roles

The port role defines the ultimate purpose of a switch port and how it handles data frames. Port roles and port states are able to transition independently of each other. RSTP uses these definitions for port roles:

  • Root port – This is the switch port on every non-root bridge that is the chosen path to the root bridge. There can only be one root port on every switch. The root port assumes the forwarding state in a stable active topology.
  • Designated port – Each segment will have at least one switch port that is the designated port for that segment. In a stable, active topology, the switch with the designated port will receive frames on the segment that are destined for the root bridge. There can only be one designated port per segment. The designated port assumes the forwarding state. All switches connected to a given segment listen to all BPDUs and determine the switch that will be the designated switch for a particular segment.
  • Alternate port – This is a switch port that offers an alternate path toward the root bridge. The alternate port assumes a discarding state in a stable, active topology. An alternate port will be present on nondesignated switches and will make a transition to a designated port if the current designated path fails.
  • Backup port – This is an additional switch port on the designated switch with a redundant link to the segment for which the switch is designated. A backup port has a higher port ID than the designated port on the designated switch. The backup port assumes the discarding state in a stable active topology.
  • Disabled port – This is a port that has no role within the operation of spanning tree.

Establishing the additional port roles allows RSTP to define a standby switch port before a failure or topology change. The alternate port moves to the forwarding state if there is a failure on the designated port for the segment.