An MST Region defines a boundary within which a single instance of STP operates. MST employs the use of Regions because not all switches in the network might run or support MST; therefore, different kinds of STPs divide the network into STP regions.
A collection of interconnected switches that have the same MST configuration comprises an MST Region. MST configuration must be the same on all switches in the same MST region; this includes the following three user-configured parameters:
- The MST Region Name (up to 32 bytes)
- Configuration Revision Number (0 to 65,535)
- VLAN-To-Instance Mapping (up to 4,096 entries)
Referencing the above, in order for two or more switches to be in the same MST Region, they must have the same VLAN-To-Instance Mapping, Configuration Revision Number, and MST Region Name. If not, the switches belong to two independent Regions. Figure 4-22 below illustrates multiple switches in a single MST Region:
Fig.4-22. MST Region Illustration
Figure 4-23 below shows the MST BPDU MST Extension field. From the output below, we can determine that the Region name is MST-Region-A and that the MST Configuration Revision Number is 0. Additionally, two Multiple Instance STPs, Instance 1 and Instance 2, have also been configured within this Region:
Fig. 4-23. MST BPDU Region, Configuration Revision, and Instance Fields
NOTE: Multiple Instance STP (MISTP) is described in detail later in this chapter.
While there is no limit to the number of MST Regions in a network, it is important to remember that a switch can belong to only a single MST Region.
MST Region Components
An MST Region consists of two different components: edge ports and boundary ports. An edge port is simply a port that connects to a non-bridging device, such as a network host. Additionally, a port that connects to a hub is also considered an edge port.
An MST boundary port connects an MST Region to a single STP Region running RSTP, 802.1D, or to another MST Region with a different MST configuration. An MST boundary port may also connect to a LAN, or to the Designated Switch that belongs to a single STP instance or another MST instance. These components are illustrated below in Figure 4-24:
Fig. 4-24. MST Components
In Figure 4-24, Switch 1 is connected to Switches 4, 5, and 6, which are all Designated Bridges that belong to a different MST Region (Switch 4) or are running RSTP (Switch 5) or legacy STP (Switch 6). Network hosts (Host 1 and Host 2) connect to the edge MST ports.
NOTE: All other ports that do not fall into any one of these two categories are simply referred to as internal MST ports. They have no special name assigned to them.