+7(495) 797-3311 www.qtech.ruМосква, Новозаводская ул., 18, стр. 1138Chapter 18 MSTP Configuration18.1 Introduction to MSTPThe MSTP (Multiple STP) is a new spanning-tree protocol which is based on the STP and theRSTP. It runs on all the bridges of a bridged-LAN. It calculates a common and internalspanning tree (CIST) for the bridge-LAN which consists of the bridges running the MSTP, theRSTP and the STP. It also calculates the independent multiple spanning-tree instances (MSTI)for each MST domain (MSTP domain). The MSTP, which adopts the RSTP for its rapidconvergence of the spanning tree, enables multiple VLANs to be mapped to the samespanning-tree instance which is independent to other spanning-tree instances. The MSTPprovides multiple forwarding paths for data traffic and enables load balancing. Moreover,because multiple VLANs share a same MSTI, the MSTP can reduce the number of spanning-tree instances, which consumes less CPU resources and reduces the bandwidth consumption.18.1.1 MSTP RegionBecause multiple VLANs can be mapped to a single spanning tree instance, IEEE 802.1scommittee raises the MST concept. The MST is used to make the association of a certainVLAN to a certain spanning tree instance.A MSTP region is composed of one or multiple bridges with the same MCID (MSTConfiguration Identification) and the bridged-LAN (a certain bridge in the MSTP region is thedesignated bridge of the LAN, and the bridges attaching to the LAN are not running STP). Allthe bridges in the same MSTP region have the same MSID.MSID consists of 3 attributes:Configuration Name: Composed by digits and lettersRevision LevelConfiguration Digest: VLANs mapping to spanning tree instancesThe bridges with the same 3 above attributes are considered as in the same MST region.When the MSTP calculates CIST in a bridged-LAN, a MSTP region is considered as a bridge.See the figure below:
