MST Region

An MST region is a group of devices that are configured together to form a logical region. The MST region presents itself to the rest of the network as a single switching device, which simplifies administration. Path cost is only incremented when traffic enters or leaves the region, regardless of the number of devices within the region. Each LAN can only be a member of one region. The following figure shows that the MST region appears as a single switching device to devices 1 and 2, but really consists of three devices.

For a switching device to be considered as part of an MST region, it must be administratively configured with the same configuration identifier information as all other devices in the MST region. The configuration identifier consists of four parts:

• Format Selector – One octet in length and is always 0. It cannot be administratively changed.
• Configuration Name – A user-assigned, case sensitive name given to the region. The maximum length of the name is 32 octets. A bridge‘s default configuration name is a character string corresponding to the bridge MAC address. This guarantees that the default behavior of a bridge is to not be part of an MST region.
• Revision Level – Two octets in length. The default value of 0 may be administratively changed.
• Configuration Digest – 16‐octet HMAC‐MD5 signature created from the configured VLAN Identification (VID)/Filtering Identification (FID) to Multiple Spanning Tree Instances (MSTI) mappings. All devices must have identical mappings to have identical configuration digests.

By default, each bridge is in its own MST region and has a default configuration name derived from the bridge MAC address. For example, if the bridge MAC address is 00-1f-45-9a-6c-b7, the default MSTP configuration name is “00:1f:45:9a:6c:b7”. When grouping two or more bridges into a single MST region, you must assign the same configuration name to each member of the region. MD5 digests are derived from a mapping of a Filtering Database ID (FID) to a Spanning Tree ID (SID), referred to as a FID-to-SID mapping (see Multiple Spanning Tree Instances (MSTI) for more information). Since there is a small probability of different mappings resulting in the same digest, the addition of administratively assigned name and version configuration ID parameters guarantee the uniqueness of the region.

SIDs exist within an MST region, each having a separate topology. Within an MST region there always exists the Internal Spanning Tree (IST) which is SID 0. There are zero or more Multiple Spanning Tree Instances (MSTIs). Each MSTI corresponds to a set of VIDs. One or more VIDs may be mapped to an SID using a FID-to-SID mapping. The IST and each MSTI may have different root bridges. Port path costs and bridge priorities may be different for each port/instance. Each bridge port has a unique port state per instance. With proper configuration, redundant links may be utilized to their maximum extent by each forwarding for one or more instances. See Configuring MSTP for more detail on how to do this.