DVS-108 Series Managed Industrial Ethernet Switches User Manual STP Status 3.3.1.2 CST Configuration Internal Spanning Tree (IST) is one of spanning trees in the MST region. Common Spanning Tree (CST) interconnects ISTs in the MST region. And Common and Internal Spanning Tree (CIST) consist of IST and CST. 3-46 Description Factory Default MST ID The ID of the MST instance. 0 VID The VLAN ID. 1 FID The filtering ID (FID). 1