In PVST+ load balancing is possible but CPU wastage and bandwidth consumption is more.
In CST(802.1q case) load balancing is not possible but CPU is not wasted.
MST makes use of the features of both the PVST+ and CST. In MST load balancing is achieved and the CPU is not wasted as well.
Normally STP is calculated for each VLAN.
So the CPU cycles needed for STP calculation is very high.
In MSTP multiple VLANS are mapped to single STP instance so that calculating the STP for each VLAN can be avoided. So in MSTP the CPU cycles will be reduced.
MST uses the concept of regions.
MST configuration consists of three attributes.
1.Configuration name which is alphanumeric (32 bytes)
2. Configuration revision number (2 bytes)
3. MST instance to VLAN mapping table.
Any two switches with same attributes belong to same region.
A switch will be at the boundary of a region if its neighbour switch which is on the same segment is on different region.
The information about the region is inserted into BPDUs and sent to the neighbor. After receiving the BPDU the neighbor switch extracts the information from the BPDU and compare s with its own. If the MST region is different then it will know that it is on the boundary of the region.
MST BPDU's (contents of MST BPDU's)-- consists of the above configuration attributes.
MST Configuration
Switch(config)#spanning-tree mode mst
Switch(config)#spanning-tree mode mst configuration
Switch(config-mst)# name <name>
Switch(config-mst)#revision <version>
Switch(config-mst)#instance <instance-id> vlan <vlan-list>