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Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
OL-4266-08
Chapter 20 Configuring STP and Prestandard IEEE 802.1s MST
Understanding How Prestandard IEEE 802.1s MST Works
•
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 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 20-4
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, and
ensures that all alternate ports and backup ports are always in the discarding state.
Rapid-PVST
Rapid-PVST uses the existing configuration for PVST+; however, Rapid-PVST uses RSTP to provide
faster convergence. Independent VLANs run their own RSTP instance.
Dynamic entries are flushed immediately on a per-port basis upon receiving a topology change.
UplinkFast and BackboneFast configurations are ignored in Rapid-PVST mode; both features are
included in RSTP.
Understanding How Prestandard IEEE 802.1s MST Works
These sections describe Multiple Spanning Tree (MST):
•
IEEE 802.1s MST Overview, page 20-15
•
MST-to-PVST Interoperability, page 20-16
•
Common Spanning Tree, page 20-18
Table 20-4 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 RSTP and MST.
No
Enabled
Listening
Discarding
No
Enabled
Learning
Learning
Yes
Enabled
Forwarding
Forwarding
Yes
Disabled
Disabled
Discarding
No