Chapter 17 Configuring Mstp - Cisco Catalyst 2975 Software Configuration Manual

Understanding MSTP
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:
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For configuration information, see the
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
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 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 65 spanning-tree instances. Instances can be
identified by any number in the range from 0 to 4094. You can assign a VLAN to only one spanning-tree
instance at a time.
IST, CIST, and CST
Unlike PVST+ and rapid PVST+ in which all the spanning-tree instances are independent, the MSTP
establishes and maintains two types of spanning trees:
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Catalyst 2975 Switch Software Configuration Guide
17-2
Multiple Spanning-Tree Regions, page 17-2
IST, CIST, and CST, page 17-3
Hop Count, page 17-6
Boundary Ports, page 17-6
IEEE 802.1s Implementation, page 17-7
"MSTP and Switch Stacks" section on page 17-9
Interoperability with IEEE 802.1D STP, page 17-9
An internal spanning tree (IST), which is the spanning tree that runs in an MST region.
Within each MST region, the MSTP maintains multiple spanning-tree instances. Instance 0 is a
special instance for a region, known as the internal spanning tree (IST). All other MST instances are
numbered from 1 to 4094.
"Configuring MSTP Features" section on page
Chapter 17 Configuring MSTP
17-15.
Figure 17-1 on page 17-4.
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