Spanning Tree Protocol: MST 802.1s

Multiple Spanning Trees (MST) is also referred to as Multiple STP (MSTP) is a standard way to use multiple instances of STP in a network using 802.1q VLANs.  Some benefits of MST include:

  • Allows configuration of STP parameters on a per-instance basis just like PVST+
  • MST instances are created by configuration, and VLANs are mapped onto them, so unlike PVST+ you don’t have redundant instances of STP running using the exact same path eating up system resources.
  • Takes all the benefits from 802.1w RSTP to retain rapid convergence.  Specifically BPDU expiry 3x Hello interval, accepting of inferior BPDUs from Designated switches, Port roles/states/types, link types, Proposal/Agreement, etc.
  • Certain catalyst platforms have a limit on the maximum number of STP instances, if decreasing the number of VLANs is not an option MST is the only solution.
  • MST is the only standards based version of STP supporting VLANs suitable for multi-vendor environments.

MSt organizes the network into regions, an MST region is a group of switches that all use MST in a consistent way, they run the same number of MST instances and map the same VLANs onto these instances.


MST uses the System ID Extension from 802.1t to embed the instance number into the Bridge ID.  The System ID extension contains 12bits, the range of MSt instance numbers is 0 – 4095, the MSt standard allows for at most 65 active MST instances.

MST uses a single BPDU to carry information about all instances and fits into a single Ethernet frame.  Ports will send BPDUs if at least one instance considers the port as designated.

Instance 0 is also referred to as the Internal Spanning Tree (IST) this MST instance serves several purposes:

  • Always exists even if no other MST instances are created
  • Without any additional configuration all VLANs are mapped to the IST
  • The IST is the only instance that interacts with STP running on switches outside of the MST region.
  • Port Roles and states are determined by the interaction of IST on region boundaries with a neighboring switch, the Role and State are inherited by ALL existing VLANs on that port, not just by VLANs mapped specifically to the IST, this is how MST regions appear as a single switch to other regions and non-MST switches.

MST blocks loops by participating in CST via the IST instance. CST determines loop free paths between regions, and is the only spanning tree that is understood and participated in by non MST switches…allowing MST to inter-operate in mixed environments.

The resulting loop free tree topology between MST regions that is created is referred to as the Common and Internal Spanning Tree (CIST)

CIST elects a CIST Root Switch by using the lowest Bridge D from all switches participating in CIST, CIST regional root switches are Root switches determined for each region. The Root port on a regional CIST Root switch is referred to as the Master port and provides connectivity from the region toward the CIST Root for all MST instances inside the region.



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