PLANET WGSW-52040 - Manual (Page 196)

22-118
Figure 22-1: Example of CIST and MST Region
In the above network, if the bridges are running the STP or the RSTP, one port between Bridge
M and Bridge B should be blocked. But if the bridges in the yellow range run the MSTP and are
configured in the same MST region, MSTP will treat this region as a bridge. Therefore, one
port between Bridge B and Root is blocked and one port on Bridge D is blocked.
MST
REGION
M
F
E
C
B
Root Root A A
D
D
22.2.1 Operations within an MSTP Region
The IST connects all the MSTP bridges in a region. When the IST converges, the root of the
IST becomes the IST master, which is the switch within the region with the lowest bridge ID
and path cost to the CST root. The IST master is also the CST root if there is only one region
within the network. If the CST root is outside the region, one of the MSTP bridges at the
boundary of the region is selected as the IST master.
When an MSTP bridge initializes, it sends BPDUs claiming itself as the root of the CST and the
IST master, with both of the path costs to the CST root and to the IST master set to zero. The
bridge also initializes all of its MST instances and claims to be the root for all of them. If the
bridge receives superior MST root information (lower bridge ID, lower path cost, and so forth)
than currently stored for the port, it relinquishes its claim as the IST master.
Within a MST region, the IST is the only spanning-tree instance that sends and receives
BPDUs. Because the MST BPDU carries information for all instances, the number of BPDUs
that need to be processed by a switch to support multiple spanning-tree instances is
significantly reduced.
All MST instances within the same region share the same protocol timers, but each MST
instance has its own topology parameters, such as root switch ID, root path cost, and so forth.