BACnet utilizes broadcast messages for certain functions, such as trying to discover BACnet devices on a network. However, these BACnet networks often incorporate IP routers to interconnect the IP infrastructure over the internet, and these routers will typically block any broadcast messages. This may cause issues for BACnet communications, such as the "Who-Is" broadcast message used to discover BACnet devices in the system and network information necessary to communicate with these devices.
If the BACnet client device/application supports static entry of network settings for all devices, then this might not be an issue. But most clients send a "Who-Is" broadcast message.
BACnet solves the IP router communication issue by utilizing a BACnet/IP Broadcast Management Device (BBMD). The BBMD will send any received broadcast messages as directed messages through the IP router to its partner BBMD devices.
For this to work, each BBMD must be configured with the IP addresses of all other BBMDs. Or, all BBMDs can send their broadcast messages to one centrally located BBMD, but all client devices must utilize this central BBMD. These entries form the BBMD device's Broadcast Distribution Table (BDT).
Many BACnet/IP devices or applications also support Foreign Device Registration (FDR). FDR allows the BACnet/IP device or application to send its broadcast messages to a BBMD. The BBMD will then forward these broadcast messages to all other BBMDs and all other FDR devices. If a subnet has only FDR supported devices, then it does not need a local BBMD. These devices can register with a BBMD on another subnet.
Connecting IP routers directly on the internet or just setting up port forwarding for direct access to BACnet devices is not a recommended practice. Contemporary Controls has created a BBMD Instruction Sheet that shows the common setup options and lists the security best practices for keeping your BACnet infrastructure safe.
View the BBMD Instruction Sheet (PDF) to learn more.