Implementing the Multiple Spanning Tree Protocol – MSTP
Interacting between MST regions and 802.1Q networks

One of the issues that arise from MST design is interoperability with the CST implementation in 802.1Q. According to the IEEE 802.1s specification, an MST switch must be able to handle at least one Internal Spanning Tree (IST).

The IST (instance 0) runs on all bridges within an MST region. An important characteristic of the IST instance is that it provides interaction at the boundary of the MST region with other MST regions and, more importantly, it is responsible for providing compatibility between the MST regions and the spanning tree of 802.1D, 802.1Q (CST) and PVST+ networks connected to the region.

The IST instance receives and sends BPDUs to the CST for compatibility with 802.1Q. The IST is capable of representing the entire MST region as a CST virtual bridge to switched networks outside the MST region.

  • MST regions appears as a single virtual bridge to the adjacent CST and MST regions. The MST region uses 802.1w port roles and operation.
  • MST switches run IST, which augments CST information with internal information about the MST region.
  • IST connects all the MST switches in the region and any CST switched domain.
  • MST establishes and maintains additional spanning trees within each MST region. These spanning trees are termed MST instances (MSTIs). The IST is numbered 0, and the MSTIs are numbered 1, 2, 3, and so on, up to 15. Any MSTI is local to the MST region and is independent of MSTIs in another region, even if the MST regions are interconnected.
  • The M-Record is a sub-field, within the BPDU of MSTIs, that contains enough information (root bridge and sender bridge priority parameters) for the corresponding instance to calculate the final topology. It does not contain any timer-related parameters (such as hello time, forward delay, and max_age) that are typically found in a regular IEEE 802.1D BPDU, as these timers are derived from the IST BPDU timers. It is important to note that within an MST region, all spanning tree instances use the same parameters as the IST.
  • MST instances combine with the IST at the boundary of MST regions to become the CST as follows:
    • M-records are always encapsulated within MST BPDUs. The original spanning trees are called "M-trees," which are active only within the MST region. M-trees merge with the IST at the boundary of the MST region and form the CST.
  • MST supports some of the PVST extensions as follows:
    • UplinkFast and BackboneFast are not available in MST mode; they are part of RSTP.
    • PortFast is supported.
    • BPDU filter and BPDU guard are supported in MST mode.
    • Loop guard and root guard are supported in MST.
    • For PVLANs, you must map a secondary VLAN to the same instance as the primary.

Each switch in the network that runs MST has a single MST configuration consisting of the following 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.