Ieee 802.1S Mst Overview - Cisco 6500 Series Software Configuration Manual

Chapter 15 Configuring STP and IEEE 802.1s MST
These sections describe Multiple Spanning Tree (MST):
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IEEE 802.1s MST Overview

Releases 12.1(11b)EX and later releases support MST. MST in this release is based on the draft version
of the IEEE standard. 802.1s for MST is an amendment to 802.1Q. MST extends the IEEE 802.1w rapid
spanning tree (RST) algorithm to multiple spanning trees. This extension provides both rapid
convergence and load balancing in a VLAN environment. MST converges faster than PVST+. MST is
backward compatible with 802.1D STP, 802.1w (rapid spanning tree protocol [RSTP]), and the Cisco
PVST+ architecture.
MST allows you to build multiple spanning trees over trunks. You can group and associate VLANs to
spanning tree instances. Each instance can have a topology independent of other spanning tree instances.
This new architecture provides multiple forwarding paths for data traffic and enables load balancing.
Network fault tolerance is improved because a failure in one instance (forwarding path) does not affect
other instances (forwarding paths).
In large networks, you can more easily administer the network and use redundant paths by locating
different VLAN and spanning tree instance assignments in different parts of the network. A
spanning tree instance can exist only on bridges that have compatible VLAN instance assignments. You
must configure a set of bridges with the same MST configuration information, which allows them to
participate in a specific set of spanning tree instances. Interconnected bridges that have the same MST
configuration are referred to as an MST region.
MST uses the modified RSTP version called the Multiple Spanning Tree Protocol (MSTP). The MST
feature has these characteristics:
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IEEE 802.1s MST Overview, page 15-15
MST-to-PVST Interoperability, page 15-16
Common Spanning Tree, page 15-18
MST Instances, page 15-18
MST Configuration Parameters, page 15-18
MST Regions, page 15-19
Message Age and Hop Count, page 15-20
Default STP Configuration, page 15-21
MST runs a variant of spanning tree called internal spanning tree (IST). IST augments the common
spanning tree (CST) information with internal information about the MST region. The MST region
appears as a single bridge to adjacent single spanning tree (SST) and MST regions.
A bridge running MST provides interoperability with single spanning tree bridges as follows:
– MST bridges run IST, which augments the common spanning tree (CST) information with
internal information about the MST region.
– IST connects all the MST bridges in the region and appears as a subtree in the CST that includes
the whole bridged domain. The MST region appears as a virtual bridge to adjacent SST bridges
and MST regions.
The common and internal spanning tree (CIST) is the collection of ISTs in each MST region,
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the CST that interconnects the MST regions, and the SST bridges. CIST is the same as an IST
inside an MST region and the same as CST outside an MST region. The STP, RSTP, and MSTP
together elect a single bridge as the root of the CIST.
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
Understanding How IEEE 802.1s MST Works
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