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14. Multiple Spanning Tree Protocol
(MSTP)
Introduction
The Multiple Spanning Tree (MST) protocol carries the concept of the IEEE 802.1w Rapid
Spanning Tree Protocol (RSTP) a leap forward by allowing you to group and associate
VLANs to multiple spanning tree instances (forwarding paths) over Link Aggregation Groups
(LAGs). Used in a VLAN environment, this added capability affords rapid convergence as
well as load balancing.
Each Multiple Spanning Tree Instance (MSTI) can have its own independent topology. The
multiplicity of forwarding paths provided by this architecture improves network fault
tolerance, because if one instance fails, data flow continues unaffected over the remaining
forwarding paths. You can manage large networks and use redundant paths more easily by
allocating different VLAN and spanning tree instance assignments in different parts of the
network.
NOTE
Terms used in this section are defined in Table 14-1.
Bridges running MST provide interoperability with Single Spanning Tree (SST) bridges, as
follows:
•
MST bridges run Internal Spanning Tree (IST). IST adds internal information about the
MST region to the Common Spanning Tree (CST) information.
•
IST connects all the MST bridges in the region and appears as a sub-tree in the CST that
includes the whole bridged domain.
•
Adjacent single Spanning Tree (SST) and MST regions regard the MST region as a single
virtual bridge.
•
The Common and Internal Spanning Tree (CIST) is the collection of the following:
Internal Spanning Trees (ISTs) in each MST region;
The Common Spanning Tree (CST) that interconnects the MST regions;
The SST bridges.
Within an MST region, CIST is identical to an IST.
Outside an MST region, CIST is identical to a CST.
The STP, RSTP, and MSTP together elect a single bridge as the root of the CIST.
Within each MST region, MST establishes and maintains MST instances (MSTIs). These are
additional spanning trees calculated by MSTP to provide a simply and fully connected active
