Alcatel-Lucent 6850-48 Network Guide
Configuring Spanning Tree Parameters
Spanning Tree Overview
OmniSwitch AOS Release 6 Network Configuration Guide
September 2009
page 13-7
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.
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.
ning Tree Algorithm examines to determine which role to assign to the 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.
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.
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:
have an enabled administrative status. The following table compares and defines 802.1D and 802.1w port
states and their associated port roles:
Role
Port/Bridge Properties
Root Port
Port connection that provides the shortest path (lowest path cost value) to the
root. The root bridge does not have a root port.
root. The root bridge does not have a root port.
Designated Port
The designated bridge provides the LAN with the shortest path to the root. The
designated port connects the LAN to this bridge.
designated port connects the LAN to this bridge.
Backup Port
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.
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.
Alternate Port
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.
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.
Disabled Port
Port is not operational. If an active connection does come up on the port, it is
assigned an appropriate role.
assigned an appropriate role.
STP Port State
RSTP Port State
Port State Definition
Port Role
Disabled
Discarding
Port is down or administratively disabled
and is not included in the topology.
and is not included in the topology.
Disabled
Blocking
Discarding
Frames are dropped, nothing is learned or
forwarded on the port. Port is temporarily
excluded from topology.
forwarded on the port. Port is temporarily
excluded from topology.
Alternate, Backup
Learning
Learning
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.
on the port and adding them to the bridge
forwarding table, but not transmitting any
data. Port is included in the active topology.
Root, Designated
Forwarding
Forwarding
Port is transmitting and receiving data and is
included in the active topology.
included in the active topology.
Root, Designated