7-20
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 7 Configuring Spanning Tree
Understanding How Multiple Spanning Tree Works
Figure 7-10
Network with Interconnected SST and MST Regions
To the spanning-tree protocol running in the SST region, an MST region appears as a single SST or
pseudobridge. Pseudobridges operate as follows:
•
The same values for root identifiers and root path costs are sent in all BPDUs of all the pseudobridge
ports. Pseudobridges differ from a single SST bridge as follows:
–
The pseudobridge BPDUs have different bridge identifiers. This difference does not affect STP
operation in the neighboring SST regions because the root identifier and root cost are the same.
–
BPDUs that are sent from the pseudobridge ports may have significantly different message ages.
Because the message age increases by 1 second for each hop, the difference in the message age
is in the order of seconds.
•
Data traffic from one port of a pseudobridge (a port at the edge of a region) to another port follows
a path that is entirely contained within the pseudobridge or MST region.
•
Data traffic belonging to different VLANs may follow different paths within the MST regions that
are established by MST.
•
Loop prevention is achieved by either of the following:
–
Blocking the appropriate pseudobridge ports by allowing one forwarding port on the boundary
and blocking all other ports.
–
Setting the CST partitions to block the ports of the SST regions.
•
A pseudobridge differs from a single SST bridge because the BPDUs that are sent from the
pseudobridge’s ports have different bridge identifiers. The root identifier and root cost are the same
for both bridges.
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
68285
