SNR S2940-8G-v2 Switch Configuration Guide
MSTP Configuration
Chapter 25
MSTP Configuration
25.1 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.
25.1.1 MSTP Region
Because multiple VLANs can be mapped to a single spanning tree instance, IEEE 802.1s com-
mittee 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:
In the above network, if the bridges are running the STP or the RSTP, one port between Bridge
M and Bridge B should be blocked. But if the bridges in the yellow range run the MSTP and are
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