Appendix A: MSTP Overview
Each MSTI functions as an independent spanning tree within a region. Consequently, each MSTI must have a root bridge to locate physical loops within the spanning tree instance. An MSTI’s root bridge is called a regional root. The MSTIs within a region may share the same regional root or they can have different regional roots.
A regional root for an MSTI must be within the region where the MSTI is located. An MSTI cannot have a regional root that is outside its region.
A regional root is selected by a combination of the MSTI Bridge Priority value and the bridge’s MAC address. The MSTI priority is analogous to the RSTP bridge priority value. Where they differ is that while the RSTP bridge priority is used to determine the root bridge for an entire bridged network, MSTI priority is used only to determine the regional root for a particular MSTI.
The range for this parameter is the same as the RSTP bridge priority; from 0 to 61,440 in sixteen increments of 4,096. To set the parameter, you select the increment that represents the desired MSTI priority value according to Table 11.
Table 11. Regional Bridge Priority Value Increments
Bridge Priority
Selections
0 |
| 32768 |
|
|
|
4096 |
| 36864 |
|
|
|
8192 |
| 40960 |
|
|
|
12288 |
| 45056 |
|
|
|
16384 |
| 49152 |
|
|
|
20480 |
| 53248 |
|
|
|
24576 |
| 57344 |
|
|
|
28672 |
| 61440 |
|
|
|
MST Region Guidelines
Following are several points to remember about regions.
A network can contain any number of regions and a region can contain any number of
The
A region can contain any number of VLANs.
All of the bridges in a region must have the same configuration name, revision level, VLANs, and VLAN to MSTI associations.
An MSTI cannot span multiple regions.
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