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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter15 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 inde pende nt o f othe r spanning -t ree
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 15-2
•IST, CIST, and CST, page 15-2
•Hop Count, page 15-4
•Boundary Ports, page 15-5
•Interoperability with 802.1D STP, page 15-5
For configuration information, see the “Configuring MSTP Features” section on page 15-11.

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 15-1 on page 15-4.
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 co nfigur ati on comm a nd, sp ec ify
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 MS T co nfigur ation; eac h m embe r
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 sp anni ng- tree inst ance s. 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:
•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 ar e
numbered from 1 to 15.
The IST is the only spanning-tree inst an ce t h at s ends and receives BPDUs; all of the other
spanning-tree instance information is contained in M-records, which are enc apsulated w ithin MSTP
BPDUs. Because the MSTP BPDU carries information for all instances, the number o f BPDUs that
need to be processed by a switch to support multiple spanning-tree instances is significantly
reduced.