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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.
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