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Chapter 22 MSTP Configuration
22.1 Introduction to MSTP
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.
22.2 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:
22-117
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