Cisco Gigabit Ethernet Switch Module for HP p-Class BladeSystem Software Configuration Guide

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Chapter 13 Configuring 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:

• Multiple Spanning-Tree Regions, page 13-2

• IST, CIST, and CST, page 13-3

• Hop Count, page 13-5

• Boundary Ports, page 13-6

• IEEE 802.1s Implementation, page 13-6

• Interoperability with IEEE 802.1D STP, page 13-8

For configuration information, see the “Configuring MSTP Features” section on page 13-14.

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 13-1 on page 13-4.

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 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.