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15-4
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 15 Understanding and Configuring Multiple Spanning Trees
Overview of MST
RSTP Port States
The port state controls the forwarding and learning processes and provides the values of discarding,
learning, and forwarding. Table 15-1 shows the STP port states and RSTP port states.
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. Figure 15-1 shows
this relationship.
Figure 15-1 Network with Interconnected SST and MST Regions
Table 15-1 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
MST
Region
MST
Region
SST
Region
SST
Region
F
F
F
F
F
F
R
F
F
F
F
F
F
F
B
B
B
B
B
r
r
r
r
r
r
r
b
b
F/f
= Forwarding
B/b = Blocking
R
= Root Bridge
r
= Root port
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