Mst Regions - Cisco 4500M Software Manual

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:
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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:
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If you connect two MST regions with different MST configurations, the MST regions do the following:
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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 boundary 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 the boundary can take up
any port role except a backup port role.
Software Configuration Guide—Release 12.2(25)EW
16-6
MST Region Overview, page 16-6
Boundary Ports, page 16-6
IST Master, page 16-7
Edge Ports, page 16-7
Link Type, page 16-7
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.
Load balance across redundant paths in the network. If two MST regions are 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.
Chapter 16 Understanding and Configuring Multiple Spanning Trees
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