MSTP (Multiple Spanning Tree Protocol)

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Multiple Spanning Tree Protocol (IEEE 802.1s) is backwards compatible with STP/RSTP and addresses the limitations of existing spanning tree protocols (STP and RSTP) in networks to include the following features:

One Common and Internal Spanning Tree (CIST) that represents the entire network's connectivity.
Grouping of multiple bridges (or switching devices) into regions that appear as one single bridge on the network.
A VLAN can be mapped to a specific Multiple Spanning Tree Instance (MSTI). MSTI allows multiple VLANs to use the same spanning tree.
Load-balancing is possible as traffic from different VLANs can use distinct paths in a region. 

Purpose

• To supplement VLAN insensitivity problem in STP/RSTP.

Edge Ports

• Directly transition to forwarding, skipping learning states. 
• Doesn’t generate topology changes when its link toggles. 
• Become a normal spanning-tree port whenever receives a BPDU.

MST Instances

• An MST bridge must be able to handle at least the following two instances 
• One Common Internal Spanning Tree (CIST) 
• One or more Multiple Spanning Tree Instance(s) (MSTIs) 
• Cisco supports 16 instances: one CIST (instance 0) and 15 MSTIs

MST Instance

• An MST Instance (MSTI) is a spanning tree instance. VLANs can be configured to run on a specific MSTI. Each created MSTI is identified by a unique number (known as an MST ID) known internally to a region. Thus an MSTI does not span across MST regions. 

Common and Internal Spanning Tree (CIST)

• A CIST represents the connectivity of the entire network and it is equivalent to a spanning tree in an STP/RSTP. The CIST is the default MST instance (MSTID 0). Any VLANs that are not members of an MST instance are members of the CIST. In an MSTP-enabled network, there is only one CIST that runs between MST regions and single spanning tree devices. A network may contain multiple MST regions and other network segments running RSTP.