Operations Within An Mstp Region; Operations Between Mst Regions - Planet XGS3-42000R User Manual

In the above network, if the bridges are running the STP or the RS TP, one port between Bridge M and Bridge
B should be blocked. But if the bridges in the yellow range run the MS TP and are configured in the same MS T
region, MSTP will treat this region as a bridge. Therefore, one port between Bridge B and Root is blocked and
one port on B ridge D is blocked.

16.1.1.1 Operations within an MSTP Region

The IS T connects all the MSTP bridges in a region. When the IS T converges, the root of the IS T bec omes the
IS T master, which is the switch within the region with the lowest bridge ID and pat h cost to the CST root. The
IS T master is also the CST root if there is only one region within the net work. If the CS T root is outside the
region, one of the MS TP bridges at the boundary of the region is selected as the IS T master.
When an MS TP bridge initializes, it sends BPDUs claiming itself as the root of the CST and the IS T master,
with both of the path costs to the CST root and to the IS T 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
IS T master. Within a MS T region, the IS T 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 prot ocol timers, but each MS T instance has its own
topology parameters, such as root switch ID, root path cost, and so forth.

16.1.1.2 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 MS T regions and all legacy STP bridges in the net work. The MS T instances combine
with the IS T at the boundary of the region to become the CS T.
The MSTI is only valid within its MST region. An MS TI has nothing to do with MS TIs in other MS T regions. The
bridges in a MS T region receive the MST BP DU of other regions through Boundary Ports. They only proc ess
CIS T related information and abandon MS TI information.
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