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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
OL-9639-06
Chapter 15 Configuring MSTP
Understanding MSTP
This chapter consists of these sections:
• Understanding MSTP, page 15-2
• Understanding RSTP, page 15-8
• Configuring MSTP Features, page 15-14
• Displaying the MST Configuration and Status, page 15-27
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 15-2
• IST, CIST, and CST, page 15-3
• Hop Count, page 15-5
• Boundary Ports, page 15-6
• IEEE 802.1s Implementation, page 15-6
• “Interoperability with IEEE 802.1D STP” section on page 15-8
For configuration information, see the “Configuring MSTP Features” section on page 15-14.
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 15-1 on page 15-4.
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 65 spanning-tree instances. You can
assign a VLAN to only one spanning-tree instance at a time.