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Figure 6-1 Example of CIST and MST Region
In the above network, if the bridges are running the STP other 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.
6.1.1.1 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 also is 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 sected 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.
6.1.1.2 Operations between MST Regions
If there are multiple regions or legacy 802.1D bridges within the network, MSTP
establishes and maintains the CST, which includes all MST regions and all legacy STP
bridges in the network. The MST instances combine with the IST at the boundary of the