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19 Spanning Tree Protocol
instance ID, and VLAN correspondence must be the same. These are set by configuration.
Trees are built separately between MST regions and within MST regions, and the topology
within an MST region can be built per MST instance.
The following explains Spanning Tree Protocols that run both between MST regions and
within MST regions.
CST
A Common Spanning Tree (CST) controls connections between MST regions, and
bridges using Single Spanning Tree. Because this topology performs calculations by
physical port as with Single Spanning Tree, it cannot perform load balancing.
IST
An Internal Spanning Tree (IST) refers to a topology that runs by default within an
MST region for connecting outside of the MST region, and for which an MST
instance ID of 0 is assigned. The port connecting outside of the MST region is called
the boundary port. Note that a unique MST instance is used to send and receive
BPDUs within and between regions. The topology information for all MST instances
is encapsulated in an MST BPDU for reporting.
CIST
A Common and Internal Spanning Tree (CIST) refers to a topology that combines
ISTs and CSTs.
The following figure provides an overview of Multiple Spanning Tree.
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