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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 7 Configuring Spanning Tree
Understanding How Multiple Spanning Tree Works
• Backup—A backup for the path that is provided by a designated port toward the leaves of the
spanning tree. Backup ports can exist only where two ports are connected together 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.
Port roles are assigned as follows:
• A root port or designated port role includes the port in the active topology.
• An alternate port or backup port role excludes the port from the active topology.
RSTP Port States
The port state controls the forwarding and learning processes and provides the values of discarding,
learning, and forwarding. Table 7-3 provides a comparison between STP port states and RSTP port
states.
In a stable topology, RSTP ensures that every root port and designated port transition to forwarding while
all alternate ports and backup ports are always in the discarding state.
MST-to-SST Interoperability
A virtual bridged LAN may contain interconnected regions of SST and MST bridges. See Figure 7-10.
Table 7-3 Comparison Between STP and RSTP Port States
Operational Status STP Port State RSTP Port State Port Included in Active Topology
Enabled Blocking
1
1. IEEE 802.1D port state designation.
Discarding
2
2. IEEE 802.1w port state designation. Discarding is the same as blocking in MST.
No
Enabled Listening Discarding No
Enabled Learning Learning Yes
Enabled Forwarding Forwarding Yes
Disabled Disabled Discarding No
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