Figure 25.1: Example of CIST and MST Region
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.
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 BP-
DUs. 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 re-
duced.
All MST instances within the same region share the same protocol timers, but each MST in-
stance has its own topology parameters, such as root switch ID, root path cost, and so forth.
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 region to become the CST.
The MSTI is only valid within its MST region. An MSTI has nothing to do with MSTIs in other
MST regions. The bridges in a MST region receive the MST BPDU of other regions through
Boundary Ports. They only process CIST related information and abandon MSTI information.
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General