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Cisco Systems IntelligentGigabit Ethernet Switch Modules for the IBMBladeCenter, Software Configuration Guide
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Chapter10 Configuring MSTP
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:
•Multiple Spanning-Tree Regions, page 10-2
•IST, CIST, and CST, page 10-2
•Hop Count, page 10-4
•Interoperability with IEEE 802.1D STP, page 10-4
For configuration information, see the “Configuring MSTP Features” section on page 10-10.

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 10-1 on page 10-7.
The MST configuration determines to which MST region each switch belongs. The configuration
includes the name of the region, the revision number, and the MST VLAN-to-instance assignme nt 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 MS T 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 only support up to spanning-tree instances. Instances can be
identified by any number in the range from 0 toYou can assign a VLAN to only one spanning-t ree
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:
•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 .
The IST is the only spanning-tree instan ce that sends and receives BPDUs. All of the ot her
spanning-tree instance information is contained in M-records, which are encapsulated within MSTP
BPDUs. Because the MSTP BPDU carries information for all instances, the number of BPDUs that
need to be processed to support multiple spanning-tree instances is significantly reduced.