?– Designated: A forwarding port elected for every switched LAN segment.
?– Alternate: An alternate path to the root bridge that is different from the path that the
root port takes.
?– Backup: A backup path that provides a redundant (but less desirable) connection to a
segment to which another switch port already connects. Backup ports can exist only
where two ports are connected in a loopback by a point-to-point link or bridge with two
or more connections to a shared LAN segment.
?– Disabled: A port that has no role within the operation of spanning tree.
Root and designated port roles include the port in the active topology. Alternate and backup
port roles exclude the port from the active topology.
The port state controls the forwarding and learning processes and provides the values of
discarding, learning, and forwarding. Table 2-12 compares STP port states with RSTP port
states.
NOTE The Cisco implementation of 802.1D includes some features that are standard in
802.1w. For example, the Cisco implementation of 802.1D determines an alternate root
port if it exists.
Table 2-12 Comparing RSTP Port States to STP
Operational Status STP Port State RSTP Port State
Port Included in
Active Topology
Enabled Blocking Discarding No
Enabled Listening Discarding No
Enabled Learning Learning Yes
Enabled Forwarding Forwarding Yes
Disabled Disabled Discarding No
Improving Performance with Spanning Tree 61
In a stable topology, RSTP ensures that every root port and designated port transitions to
forwarding, while all alternate ports and backup ports are always in the discarding state.
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