Common and Internal Spanning Tree (CIST)

MSTP uses all Spanning Tree region information to create a single Common and Internal Spanning Tree (CIST) that represents the connectivity of the entire network. This is equivalent to the single Spanning Tree used for STP and RSTP.

The MSTP enabled network may contain any combination of Single Spanning Tree (SST) regions and Multiple Spanning Tree (MST) regions. A typical network may contain multiple MST regions as well as separate LAN segments running legacy STP and RSTP Spanning Tree protocols. The CIST contains a root bridge, which is the root of the Spanning Tree for the network. The CIST root may be, but is not necessarily, located inside an MST region. Each MST region contains a CIST regional root which may be the CIST root if the CIST root is internal to the region. If the CIST root is external to the region, the CIST regional root provides the connectivity to the CIST root. Bridges in an MSTP topology compare their received BPDUs to calculate their shortest path to the CIST root, CIST regional root, and MSTI regional root.

Ideally, there should be one all-encompassing region. This is not always possible, for example, when non-MSTP bridges exist such as those shown in Multiple Spanning Tree Overview. From the outside, the region appears as a single Spanning Tree bridge which is part of the Common Spanning Tree (CST). A port which connects to a bridge not having the same MST configuration ID, or which is not running MSTP, forms part of the boundary of the region. The region attaches to the CST at the root port of the CIST regional root. All other region boundary ports which provide paths to the root are alternate ports and remain blocking until the topology changes, causing a new regional root port to be chosen. Ports which provide a path to the root for other bridges at the region boundary are designated ports. At boundary ports, port states for MSTIs follow the states of the CIST for the port.