Mst Regions; Boundary Ports - Cisco 6500 Series Software Configuration Manual

Chapter 15 Configuring STP and IEEE 802.1s MST

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 process assigns a port role to
the boundary and assigns the same state as the state of the IST port. The IST port at the boundary can
take up any port role except a backup port role.
IST Master
The IST master of an MST region is the bridge with the lowest bridge identifier and the least path cost
to the CST root. If an MST bridge is the root bridge for CST, then it is the IST master of that MST region.
If the CST root is outside the MST region, then one of the MST bridges at the boundary is selected as
the IST master. Other bridges on the boundary that belong to the same region eventually block the
boundary ports that lead to the root.
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MST Region Overview, page 15-19
Boundary Ports, page 15-19
IST Master, page 15-19
Edge Ports, page 15-20
Link Type, page 15-20
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.
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
Understanding How IEEE 802.1s MST Works
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