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Chapter 6 MSTP Configuration
6.1 MSTP Introduction
The MSTP (Multiple STP) is a new spanning-tree protocol which is based on the STP
and the RSTP. It runs on all the bridges of a bridged-LAN. It calculates a common and
internal spanning tree (CIST) for the bridge-LAN which consists of the bridges running the
MSTP, the RSTP 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 rapid convergence of the spanning tree, enables multiple VLANs to be
mapped to the same spanning-tree instance which is independent to other spanning-tree
instances. The MSTP provides 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.
6.1.1 MSTP Region
Because multiple VLANs can be mapped to a single spanning tree instance, IEEE
802.1s committee raises the MST concept. The MST is used to make the association of a
certain VLAN to a certain spanning tree instance.
A MSTP region is composed of one or multiple bridges with the same MCID (MST
Configuration Identification) and the bridged-LAN (a certain bridge in the MSTP region is
the designated bridge of the LAN, and the bridges attaching to the LAN are not running
STP). All the bridges in the same MSTP region have the same MSID.
MSID consists of 3 attributes:
Configuration Name: Composed by digits and letters
Revision Level
Configuration Digest: VLANs mapping to spanning tree instances
The 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: