Specifications

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Cisco MWR 2941 Mobile Wireless Edge Router Release 3.5 Software Configuration Guide, Cisco IOS Release 15.1(3)MR
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Chapter 10 Configuring MSTP
Understanding MSTP
• Configuring MSTP Features, page 10-13
• Displaying the MST Configuration and Status, page 10-26
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.
• Multiple Spanning-Tree Regions, page 10-2
• IST, CIST, and CST, page 10-2
• Hop Count, page 10-5
• Boundary Ports, page 10-6
• IEEE 802.1s Implementation, page 10-6
• “Interoperability with IEEE 802.1D STP” section on page 10-8
For configuration information, see the “Configuring MSTP Features” section on page 10-13.
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 10-4.
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 router for a region by using the spanning-tree mst configuration global configuration
command, after which the router 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.
IST, CIST, and CST
Unlike 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 4094.