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Chapter 8 MSTP Configuration
8.1 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.
8.1.1 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:
