Multiple Spanning-Tree maps the set of VLANs into MST instances (MSTI) which each has an instance-specific STP root switch. In addition, there is a region Internal Spanning Tree (IST) aka MSTI0 that is used for exchanging MSTP BPDUs for all MSTIs. IST BPDUs (capture 1-1) carries all the STP information inside an MSTP Region.
First, MSTP BPDU includes information related to IST such as switch Bridge Id, Root Bridge Id for the Common and Internal Spanning Tree Root (CIST Root) and a timer values (Max Age, Hello Time and Forward Delay). The timer values are used in each MSTP Instances.
Second, The MSTP BPDU carries an MST extension header that includes the name of the MST Region, its Config revision number and a hash value. The hash value is derived from the VLAN to MSTI mapping information, the actual 1:1 VLAN to MSTI information is not carried within BPDU packets. There is also information about the CIST Regional (Internal) Root switch. The difference between the CIST Root and the CIST Regional Root is that the CIST Root is used as an STP Root for all regions when there are multiple MSTP regions connected with each other. The CIST Regional Root in turns is used as an MST Region IST root. MST Extension header carries M-records, which contains the MST Instance specific information such as MSTI Regional Root that is used to create Instance specific loop-free Layer 2 path inside a region. The root election process is based on the Proposal/Agreement messages just like in the Rapid PVST+/RSTP.
