Foreign Device Registration (FDR) lets a BACnet/IP device located on a different IP sub-network register with a BBMD to receive the BACnet broadcasts of the BBMD's sub-network.

A Foreign Device is a BACnet/IP node that wants to listen to another IP sub-network without being physically attached to it. The workflow is: the Foreign Device sends a Register-Foreign-Device message to the target sub-network's BBMD, with a TTL (typically 60-300 s); the BBMD adds the Foreign Device to its Foreign Device Table (FDT); on every subsequent broadcast (Who-Is, etc.), the BBMD forwards the message to the Foreign Device; and the Foreign Device must re-register before the TTL expires. The classic use case is a remote BMS supervisor talking to controller sub-networks on separate VLANs.

Set the TTL so that re-registration happens at roughly one third of the lifetime: a TTL just barely longer than the network's worst-case latency leads to silent supervisor blackouts.