Multiple Spanning Tree
The original 802.1D only supports 1 instance of STP for the whole of the network. All VLANs had the same topology. This means that all VLANs would have to follow the same path through the network(no load balancing). PVST allowed for a per VLAN spanning tree topology.
MSTI: multiple spanning tree instance IST: internal spanning tree
Cisco supports up to 16 instances of MST. Configuration is as follows
Conf t
Spanning-tree mode mst
Spanning-tree mst 0 root primary
Spanning-tree mst 1 root primary
Spanning-tree mst 2 root primary
Spanning-tree mst configuration
Name Ent_Core
Revision 2
Instance 1 vlan 10, 20
Instance 2 vlan 99
This config would create 3 instances, instance 2 holding VLAN 99, instance 1 holding VLANs 10 and 20, and instance 0 holding the remainder of VLANs
Common misconfiguration issues are VLAN assignment to a IST and trunk pruning
MST region boundary is any port that connects to another MST region, 802.1D or 802.1W BPDU. MST switches can detect PVST+ at a region boundary
MST as the root bridge that all region boundaries flood the same IST instance to all VLANs in the PVST topology. This allows the PVST switch to detect the alternate link and block the port on its end.
MST is not the root bridge. There is no option to load balance traffic as all IST’s must remain consistent.