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.