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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter15 Understanding and Configuring Multiple Spanning Trees
Overview of MST
MST BPDUs contain the MST configuration ID and the checksum. An MST bridge accepts an MST
BPDU only if the MST BPDU configuration ID and the checksum match its own MST region
configuration ID and checksum. If either value is different, the MST BPDU is considered to be an
SST BPDU.
MST Regions
These sections describe MST regions:
•MST Region Overview, page 15-6
•Boundary Ports, page 15-6
•IST Master, page 15-7
•Edge Ports, page 15-7
•Link Type, page 15-7

MST Region Overview

Interconnected bridges that have the same MST configuration are referred to as an MST region. There
is no limit on the number of MST regions in the network.
To form an MST region, bridges can be either of the following:
•An MST bridge that is the only member of the MST region .
•An MST bridge interconnected by a LAN. A LAN’s designated bridge has the same MST
configuration as an MST bridge. All the bridges on the LAN can process MST BPDUs.
If you connect two MST regions with different MST configurations, the MST regions do the following:
•Load balance across redundant paths in the network. If two MST regions ar e redundantly connected,
all traffic flows on a single connection with the MST regions in a network.
•Provide an RSTP handshake to enable rapid connectivity between regions. However, the
handshaking is not as fast as between two bridges. To prevent loops, all the bridges inside the region
must agree upon the connections to other regions. This situation introduces a delay. We do not
recommend partitioning the network into a large number of regions.

Boundary Ports

A boundary port is a port that connects to a LAN, the designated bridge of which is either an SST bridge
or a bridge with a different MST configuration. A designated port knows that it is on the bo undary if it
detects an STP bridge or receives an agreement message from an RST or MST bridge with a different
configuration.
At the boundary, the role of MST ports do not matter; their state is forced to be the same as the IST port
state. If the boundary flag is set for the port, the MSTP Port Role selection mechanism assigns a port
role to the boundary and the same state as that of the IST port. The IST port at t he boundary can take up
any port role except a backup port role.