1-11Basic Concepts in MSTPFigure 1-4 Basic concepts in MSTPCSTRegion A0VLAN 1 mapped to instance 1VLAN 2 mapped to instance 2Other VLANs mapped to CISTRegion B0VLAN 1 mapped to instance 1VLAN 2 mapped to instance 2Other VLANs mapped to CISTRegion C0VLAN 1 mapped to instance 1VLAN 2 and 3 mapped toinstance 2Other VLANs mapped to CISTRegion D0VLAN 1 mapped to instance 1,B as regional root bridgeVLAN 2 mapped to instance 2,C as regional root bridgeOther VLANs mapped to CISTBPDU BPDUBPDUCDBAAssume that all devices in Figure 1-4 are running MSTP. This section explains some basic concepts ofMSTP.MST regionA multiple spanning tree region (MST region) consists of multiple devices in a switched network and thenetwork segments among them. These devices have the following characteristics:z All are MSTP-enabled,z They have the same region name,z They have the same VLAN-to-instance mapping configuration,z They have the same MSTP revision level configuration, andz They are physically linked with one another.For example, all the devices in region A0 in Figure 1-4 have the same MST region configuration:z The same region name,z The same VLAN-to-instance mapping configuration (VLAN 1 is mapped to MSTI 1, VLAN 2 toMSTI 2, and the rest to the common and internal spanning tree (CIST, that is, MSTI 0), andz The same MSTP revision level (not shown in the figure).Multiple MST regions can exist in a switched network. You can use an MSTP command to assignmultiple devices to the same MST region.
