Cisco Systems 4500 MST-to-SST Interoperability

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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 18 Configuring STP and MST
About MST
RSTP Port Roles
In RSTP, the port roles are defined as follows:
• Root—A forwarding port elected for the spanning tree topology.
• Designated—A forwarding port elected for every switched LAN segment.
• Alternate—An alternate path to the root bridge to that provided by the current root port.
• Backup—A backup for the path 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 mode 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.
The system assigns port roles 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 18- 5 shows the STP port states and RSTP port states.
In a stable topology, RSTP ensures that every root port and designated port transitions to th e forwarding
state 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. Figure 18-2 shows
this relationship.
Table 18-5 Comparison Between STP and RSTP Port States
Operational Status STP Port State RSTP Port State Port Included in Active Topology
Enabled Blocking1
1. IEEE 802.1D port state designation.
Discarding2
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