Cisco 7604 Configuration Manual page 291

Chapter 20 Configuring STP and Prestandard IEEE 802.1s MST
Figure 20-8 Network with Interconnected SST and MST Regions
SST
Region
F/f = Forwarding
B/b = Blocking
R = Root Bridge
r = Root port
An MST region appears as an SST or pseudobridge to STP running in the SST region. 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:
–
–
Data traffic from one port of a pseudobridge (a port at the edge of a region) to another port follows
•
a path entirely contained within the pseudobridge or MST region.
Data traffic belonging to different VLANs may follow different paths within the MST regions
•
established by MST.
Loop prevention is achieved by either of the following:
•
–
–
A pseudobridge differs from a single SST bridge because the BPDUs sent from the pseudobridge's
•
ports have different bridge identifiers. The root identifier and root cost are the same for both bridges.
These guidelines apply in a topology where you configure MST switches (all in the same region) to
interact with PVST+ switches:
Configure the root for all VLANs inside the MST region as shown in this example:
•
Router# show spanning-tree mst interface gigabitethernet 1/1
GigabitEthernet1/1 of MST00 is root forwarding
OL-4266-08
MST
Region
r
B
F
F
F
r
b r
F
r
F
F
F
R
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 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.
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.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
Understanding How Prestandard IEEE 802.1s MST Works
B
B
B
F
F
r
F
B
F
F
MST
Region
F
r
r
SST
b
Region
20-17