Alcatel-Lucent OmniSwitch 6850-48 Network Configuration Manual page 309

Configuring Spanning Tree Parameters
During the process of calculating the Spanning Tree topology, each port on every bridge is assigned a port
role based on how the port and/or its bridge will participate in the active Spanning Tree topology.
The following table provides a list of port role types and the port and/or bridge properties that the Span-
ning Tree Algorithm examines to determine which role to assign to the port.
Role
Root Port
Designated Port
Backup Port
Alternate Port
Disabled Port
Note. The distinction between a backup port and an alternate port was introduced with the IEEE 802.1w
standard to help define rapid transition of an alternate port to a root port.
The role a port plays or may potentially play in the active Spanning Tree topology determines the port's
operating state; discarding, learning, or forwarding. The port state is also configurable in that it is possi-
ble to enable or disable a port's administrative status and/or specify a forwarding or blocking state that is
only changed through user intervention.
The Spanning Tree Algorithm only includes ports in its calculations that are operational (link is up) and
have an enabled administrative status. The following table compares and defines 802.1D and 802.1w port
states and their associated port roles:
STP Port State RSTP Port State
Disabled Discarding
Blocking Discarding
Learning Learning
Forwarding Forwarding
OmniSwitch AOS Release 6 Network Configuration Guide
Port/Bridge Properties
Port connection that provides the shortest path (lowest path cost value) to the
root. The root bridge does not have a root port.
The designated bridge provides the LAN with the shortest path to the root. The
designated port connects the LAN to this bridge.
Any operational port on the designated bridge that is not a root or designated
port. Provides a backup connection for the designated port. A backup port can
only exist when there are redundant designated port connections to the LAN.
Any operational port that is not the root port for its bridge and its bridge is not
the designated bridge for the LAN. An alternate port offers an alternate path to
the root bridge if the root port on its own bridge goes down.
Port is not operational. If an active connection does come up on the port, it is
assigned an appropriate role.
Port State Definition
Port is down or administratively disabled
and is not included in the topology.
Frames are dropped, nothing is learned or
forwarded on the port. Port is temporarily
excluded from topology.
Port is learning MAC addresses that are seen
on the port and adding them to the bridge
forwarding table, but not transmitting any
data. Port is included in the active topology.
Port is transmitting and receiving data and is
included in the active topology.
September 2009
Spanning Tree Overview
Port Role
Disabled
Alternate, Backup
Root, Designated
Root, Designated
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