User's Manual

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17-2

Figure 17-1 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.

17.1.1.1 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 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.

17.1.1.2 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 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.

MST

REGION

Root