In a case of an IP subnet where only a few (or perhaps just one) BACnet/IP device exists, a local BBMD may be considered excessive for BACnet broadcast message support. An alternative for that subnet is for each BACnet/IP device to register as “foreign device” with a particular BBMD on a remote subnet. Once registered, the device is added to that BBMD’s FDT (Foreign Device Table). It then becomes that BBMD’s responsibility to deliver global BACnet broadcast messages to that remote device.

Because this scheme is sometimes used for BACnet/IP devices that are not “permanent,” it was designed with a mandatory “registration lifetime” feature. When any BACnet/IP device registers as a foreign device with a BBMD, it must specify its “Time-to-Live” value, in seconds. It is then expected to re-register within this period, else the BBMD will remove (purge) it from its FDT. This prevents unneccesary broadcast delivery attempts to “part-time participants.”

The FDT in any BBMD reflects the current list of its registered foreign devices, along with each device’s Time-to-Live value and calculated purge time. If the NiagaraAX station is configured as a BBMD, you can see all entries in its FDT. See Foreign Device Table notes.

The “foreign device” term implies no stigma—it is purely BBMD-centric. The most expedient configuration for a Niagara station may well be as a foreign device, providing its local IP subnet has no BBMD and other BACnet/IP devices (if any) are currently each registered as foreign device with a remote BBMD.Unlike a BBMD’s broadcast distribution table, which is identical in each BBMD, the foreign device table in each BBMD is unique to that BBMD.