In MNB-1000 (Plant Controller) firmware Revision 1.50E, the Spanning Tree Protocol is Disabled, by default.
All previous versions were shipped with the Spanning Tree Protocol enabled.
I/A Series MNB-1000 (Plant Controller), firmware Revision 1.50E and later.
In some cases where multiple BACnet routers (eg, MNB-1000, UNC, ENC, etc) were installed on the same BACnet-IP internetwork, having the Spanning Tree Protocol enabled in the MNB-1000 would allow a "router loop".
Be aware that in MNB-1000s with firmware Revision 1.50E and later, the Spanning Tree protocol is disabled, by default.
Click here to see the Release Notes for firmware Revision 1.50E .