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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 7 Configuring Spanning Tree
Understanding How Multiple Spanning Tree Works
MST Region
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 that is 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 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 certain delay. We do
not recommend partitioning the network into a large number of regions.
Switches running software release 8.3(1) and later releases form a different region than that of
neighboring switches running earlier releases.
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 does not matter; the MST-port 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.
CIST Regional Root
The CIST regional root 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 CIST regional root
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 CIST regional root. Other bridges on the boundary that belong to the same
region eventually block the boundary ports that lead to the root.
If two or more bridges at the boundary of a region have an identical path to the root, you can set a slightly
lower bridge priority (higher port priority number) to make a specific bridge the CIST regional root.
The root path cost and message age inside a region stay constant, but the IST path cost is incremented
and the IST remaining hops are decremented at each hop. Enter the
show spantree mst
command to
display the information about the CIST regional root, path cost, and remaining hops for the bridge.
