MSTP

Multiple Spanning Tree Protocol:

• Configures separate spanning tree for each VLAN group, Defines an extension to RSTP.All other alternate paths are blocked within each spanning tree.
• MSTP creates MST regions, that run multiple MSTI (MST Instances). Regions and different STP bridges are interconnected with one CST (common spanning tree).
• MSTP was inspired by Cisco's MISTP - Multiple Instances Spanning tree protocol.
• MSTP includes all spanning tree information in one single BPDU, thereby reducing the number of BPDUs in a LAN communicating STP info for each VLAN. It also has backwards compatibility with RSTP and STP - This is achieved by adding additional region info after standard RSTP BPDU, also adding a number of MSTI messages (0-64). Each MSTI message conveys STP info for each instance. The instance is associated with a number of VLANs, the frames inside the VLANS operate in the Spanning tree instance within a MST region. A MST region is determined by other MSTP bridges with a MD5 digest in their VLAN instance table in the MSTP BPDU.
• If an RSTP bridge see's a MSTP BPDU, it is seen as a RSTP BPDU (backwards compatible), therefore RSTP bridges see the a MSTP region as a single RSTP bridge, no matter how many MSTP bridges are inside the region. Another measure to see a single MST region is to incorporate a protocol that uses remaining hops as a TTL counter instead of a message age used by RSTP. The age increments only when STP info enters a MST region, therefore one region is one hop. Edge ports are known as boundary port, and can be configured to rapidly change to forwarding when connected to endpoints.