User's Manual

17-1
Chapter 17 MSTP Configuration
17.1 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.
17.1.1 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: