About Mst - Cisco Catalyst 4500 Series Software Configuration Manual

About MST

About MST
The following sections describe how MST works on a Catalyst 4500 series switch:
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IEEE 802.1s MST
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 per-VLAN Spanning Tree Plus (PVST+) and 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 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.
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, MSTP. MST has the following characteristics:
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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
23-22
IEEE 802.1s MST, page 23-22
IEEE 802.1w RSTP, page 23-23
MST-to-SST Interoperability, page 23-24
Common Spanning Tree, page 23-25
MST Instances, page 23-26
MST Configuration Parameters, page 23-26
MST Regions, page 23-26
Message Age and Hop Count, page 23-28
MST runs a variant of spanning tree called Internal Spanning Tree (IST). IST augments 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 SST bridges as follows:
– MST bridges run IST, which augments 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
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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 the following: ISTs in each
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MST region, the CST that interconnects the MST regions, and the SST bridges. CIST is
identical to an IST inside an MST region and identical to a CST outside an MST region. The
STP, RSTP, and MSTP together elect a single bridge as the root of the CIST.
Chapter 23 Configuring STP and MST