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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter15 Understanding and Configuring Multiple Spanning Trees
Overview of MST
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+ architectur e.
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 othe r 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 networ k and use redundant paths by locating
different VLAN and spanning tree instance assignments in different p arts of the network. A
spanningtree 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:
•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:
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MST bridges run IST, which augments CST information with internal information about the
MST region.
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IST connects all the MST bridges in the region and app ears as a subtree in the CST that inclu des
the whole bridged domain. The MST region appears as a virtual brid ge to adjacent SST bridges
and MST regions.
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The Common and Internal Spanning Tree (CIST) is the collection of the following: ISTs in each
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.
•MST establishes and maintains additional spanning trees within each MST region. These spanning
trees are termed MST instances (MSTIs). The IST is numbered 0, and the MSTIs are number ed 1,
2, 3, and so on. Any MSTI is local to the MST region and is independent of MSTIs in anot her region,
even if the MST regions are interconnected.
MST instances combine with the IST at the boundary of MST regions to become the CST as fol lows:
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Spanning tree information for an MSTI is contained in an MSTP record (M-record).
M-records are always encapsulated within MST bridge protocol data units (BPDUs). The
original spanning trees computed by MSTP are called M-trees, which are active only within the
MST region. M-trees merge with the IST at the boundary of the MST region and form the CST.
•MST provides interoperability with PVST+ by generating PVST+ BPDUs for the non-CST VLANs.
•MST supports some of the PVST+ extensions in MSTP as follows:
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UplinkFast and BackboneFast are not available in MST mode; they are part of RSTP.
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PortFast is supported.
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BPDU filter and BPDU guard are supported in MST mode.
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Loop guard and root guard are supported in MST. MST preserves the VLAN 1 disabled
functionality except that BPDUs are still transmitted in VLAN 1.