Implementing the Multiple Spanning Tree Protocol – MSTP
MST regions

MST differs from the other spanning tree implementation in that it combines some, but not necessarily all, VLANs into logical spanning tree instances. This raises the problem of determining what VLAN is to be associated with what instance. More precisely, tagging BPDUs so that receiving devices can identify the instances and the VLANs to which they apply.

The issue is irrelevant in the case of the 802.1Q standard, where all instances are mapped to a unique and common instance Common Spanning Tree (CST). In the PVST+ implementation, different VLANs carry the BPDUs for their respective instance (one BPDU per VLAN) based on the VLAN tagging information.

To provide this logical assignment of VLANS to spanning trees, each switch running MST in the network has a single MST configuration that consists of three attributes:

  • An alphanumeric configuration name (32 bytes)
  • A configuration revision number (two bytes)
  • A 4096-element table that associates each of the potential 4096 VLANs supported on the chassis to a given instance

To be part of a common MST region, a group of switches must share the same configuration attributes. It is up to the network administrator to properly propagate the configuration throughout the region.

NOTE:

If two switches differ on one or more configuration attributes, they are part of different regions.

To ensure a consistent VLAN-to-instance mapping, it is necessary for the protocol to be able to exactly identify the boundaries of the regions. For that purpose, the characteristics of the region are included in BPDUs. The exact VLANs-to-instance mapping is not propagated in the BPDU, because the switches only need to know whether they are in the same region as a neighbor.

Therefore, only a digest of the VLANs-to-instance mapping table is sent, along with the revision number and the name. Once a switch receives a BPDU, it extracts the digest (a numerical value derived from the VLAN-to-instance mapping table through a mathematical function) and compares it with its own computed digest. If the digests differ, the mapping must be different, so the port on which the BPDU was received is at the boundary of a region.

In generic terms, a port is at the boundary of a region if the designated bridge on its segment is in a different region or if it receives legacy 802.1D BPDUs. In the figure, the port on B1 is at the boundary of region A, whereas the ports on B2 and B3 are internal to region B.