Mst Overview - Cisco 7604 Configuration Manual

Understanding MST
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MST Overview

MST maps multiple VLANs 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. MST improves the fault tolerance of the network because a failure
in one instance (forwarding path) does not affect other instances (forwarding paths).
The most common initial deployment of MST is in the backbone and distribution layers of a Layer 2
switched network. This deployment provides the kind of highly available network that is required in a
service-provider environment.
MST provides rapid spanning tree convergence through explicit handshaking, which eliminates the
802.1D forwarding delay and quickly transitions root bridge ports and designated ports to the forwarding
state.
MST improves spanning tree operation and maintains backward compatibility with these STP versions:
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For information about PVST+ and rapid PVST+, see
IEEE 802.1s MST."
root guard, and so forth, see
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MST Regions
For routers to participate in MST instances, you must consistently configure the routers with the same
MST configuration information. A collection of interconnected routers that have the same MST
configuration comprises an MST region as shown in
The MST configuration controls to which MST region each router belongs. The configuration includes
the name of the region, the revision number, and the MST VLAN-to-instance assignment map.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
19-2
Hop Count, page 19-6
Boundary Ports, page 19-6
Standard-Compliant MST Implementation, page 19-7
Interoperability with IEEE 802.1D-1998 STP, page 19-9
Original 802.1D spanning tree
Existing Cisco-proprietary Multiple Instance STP (MISTP)
Existing Cisco per-VLAN spanning tree plus (PVST+)
Rapid per-VLAN spanning tree plus (rapid PVST+)
For information about other spanning tree features such as Port Fast, UplinkFast,
Chapter 21, "Configuring Optional STP Features."
IEEE 802.1w defined the Rapid Spanning Tree Protocol (RSTP) and was incorporated into
IEEE 802.1D.
IEEE 802.1s defined MST and was incorporated into IEEE 802.1Q.
Chapter 19 Configuring Standard-Compliant IEEE MST
Chapter 20, "Configuring STP and Prestandard
Figure 19-1 on page 19-5.
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