MSTP (Multiple Spanning Tree Protocol) logically divides a Layer 2 network into regions. The Common and Internal Spanning Tree (CIST) is a single spanning tree domain that interconnects MSTP regions. The CIST is responsible for creating a loop-free topology by exchanging and propagating BPDUs across regions to form a Common Spanning Tree (CST).
In essence, the CIST is similar to having a large spanning tree across the entire network. The CIST has its own root bridge that is common to all MSTP regions, and each MSTP region elects a CIST regional root that connects that region to the CIST, thereby forming a CST.
The switch assigns the CIST an instance ID of 0, which allows the CIST to send BPDUs for itself in addition to all of the MSTIs within an MSTP region. Inside a region, the BPDUs contain CIST records and piggybacked M-records. The CIST records contain information about the CIST, and the M-records contain information about the MSTIs. Boundary ports exchange only CIST record BPDUs.
All MSTP configurations require a CIST domain. You must first configure the CIST domain before configuring any MSTIs. By default, all MSTI (Multiple Spanning Tree Instances) ports in the region are inherited by the CIST. You cannot delete or disable a CIST if any of the MSTIs are active in the system.
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