Westermo MRI-128-F4G-PSE24 - Manual (Page 79)

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A Common Spanning Tree (CST) interconnects all adjuacent MST regions and acts
as a virtual bridge node for communications with STP or RSTP nodes in the global
network. MSTP connects all bridges and LAN segments with a single Common and
Internal Spanning Tree (CIST). The CIST is formed as a result of the running
spanning tree algorithm between switches that support the STP, RSTP, MSTP
protocols.
The figure shows the CST large network. In this network, a Region may has
different instances and its own forwarding path and table, however, it acts as a
single Brige of CST.