Rstp Port Roles - Cisco Catalyst 4500 Series Configuration Manual

About MST

RSTP Port Roles

In RSTP, the port roles are defined as follows:
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•
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The system assigns port roles as follows:
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RSTP Port States
The port state controls the forwarding and learning processes and provides the values of discarding,
learning, and forwarding.
Table 20-5
Operational Status
Enabled
Enabled
Enabled
Enabled
Disabled
1. IEEE 802.1D port state designation.
2. IEEE 802.1w port state designation. Discarding is the same as blocking in MST.
In a stable topology, RSTP ensures that every root port and designated port transitions to the 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.
this relationship.
Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
20-24
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.
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.
Table 20-5
Comparison Between STP and RSTP Port States
STP Port State
Blocking
Listening
Learning
Forwarding
Disabled
shows the STP port states and RSTP port states.
RSTP Port State
1 2
Discarding
Discarding
Learning
Forwarding
Discarding
Chapter 20 Configuring STP and MST
Port Included in Active Topology
No
No
Yes
Yes
No
Figure 20-2 shows
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