18-2
Catalyst 3750 Switch Software Configuration Guide
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Chapter 18 Configuring MSTP
Understanding MSTP
• Configuring MSTP Features, page 18-12
• Displaying the MST Configuration and Status, page 18-24
Understanding MSTP
MSTP, which uses RSTP for rapid convergence, enables VLANs to be grouped into a spanning-tree
instance, with each instance having a spanning-tree topology independent of other spanning-tree
instances. This architecture provides multiple forwarding paths for data traffic, enables load balancing,
and reduces the number of spanning-tree instances required to support a large number of VLANs.
These sections describe how the MSTP works:
• Multiple Spanning-Tree Regions, page 18-2
• IST, CIST, and CST, page 18-3
• Hop Count, page 18-5
• Boundary Ports, page 18-5
• “MSTP and Switch Stacks” section on page 18-6
• “Interoperability with 802.1D STP” section on page 18-6
For configuration information, see the “Configuring MSTP Features” section on page 18-12.
Multiple Spanning-Tree Regions
For switches to participate in multiple spanning-tree (MST) instances, you must consistently configure
the switches with the same MST configuration information. A collection of interconnected switches that
have the same MST configuration comprises an MST region as shown in Figure 18-1 on page 18-4.
The MST configuration controls to which MST region each switch belongs. The configuration includes
the name of the region, the revision number, and the MST VLAN-to-instance assignment map. You
configure the switch for a region by using the spanning-tree mst configuration global configuration
command, after which the switch enters the MST configuration mode. From this mode, you can map
VLANs to an MST instance by using the instance MST configuration command, specify the region name
by using the name MST configuration command, and set the revision number by using the revision MST
configuration command.
A region can have one member or multiple members with the same MST configuration; each member
must be capable of processing RSTP bridge protocol data units (BPDUs). There is no limit to the number
of MST regions in a network, but each region can support up to 16 spanning-tree instances. You can
assign a VLAN to only one spanning-tree instance at a time.
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