1-11Figure 1-4 Basic MSTP terminologiesCSTBPDURegion A0:VLAN 1 mapped to MSTI 1VLAN 2 mapped to MSTI 2Other VLANs mapped to CISTBPDU BPDUADCBRegion B0:VLAN 1 mapped to MSTI 1VLAN 2 mapped to MSTI 2Other VLANs mapped to CISTRegion C0:VLAN 1 mapped to MSTI 1VLAN 2 and 3 mapped to MSTI 2Other VLANs mapped to CISTRegion D0:VLAN 1 mapped to MSTI 1, Bas the regional root bridgeVLAN 2 mapped to MSTI 2, Cas the regional root bridgeOther VLANs mapped to CISTMST regionA multiple spanning tree region (MST region) comprises multiple physically-interconnectedMSTP-enabled switches and the corresponding network segments connected to these switches. Theseswitches have the same region name, the same VLAN-to-MSTI mapping configuration and the sameMSTP revision level.A switched network can contain multiple MST regions. You can group multiple switches into one MSTregion by using the corresponding MSTP configuration commands.As shown in Figure 1-4, all the switches in region A0 are of the same MST region-related configuration,including:z Region namez VLAN-to-MSTI mapping (that is, VLAN 1 is mapped to MSTI 1, VLAN 2 is mapped to MSTI 2, andthe other VLANs are mapped to CIST.)z MSTP revision level (not shown in Figure 1-4)MSTIA multiple spanning tree instance (MSTI) refers to a spanning tree in an MST region.Multiple spanning trees can be established in one MST region. These spanning trees are independentof each other. For example, each region in Figure 1-4 contains multiple spanning trees known as MSTIs.Each of these spanning trees corresponds to a VLAN.VLAN-to-MSTI mapping tableA VLAN-to-MSTI mapping table is maintained for each MST region. The table is a collection ofmappings between VLANs and MSTIs. For example, in Figure 1-4, the VLAN-to-MSTI mapping table ofregion A0 contains these mappings: VLAN 1 to MSTI 1; VLAN 2 to MSTI 2, and other VLANs to CIST. Inan MST region, load balancing is implemented according to the VLAN-to-MSTI mapping table.
