October 2011 - When attaching BACnet devices to a sub-netted IP network, you will need a BBMD (BACnet/IP Broadcast Management Device) – since IP routers block broadcast messages. The BBMD encapsulates a broadcast message so it can pass as a directed message from one subnet to another – since directed messages pass through IP routers. At the destination, a BBMD device decodes the message and broadcasts it to its local subnet. You may not need a BBMD in a subnet if all the subnet's BACnet/IP devices support FDR (Foreign Device Registration), but in a sub-netted network you will need at least one BBMD to register FDR devices.
In the example, the Skorpion EIPR IP Router's public port connects to the enterprise network (and the Internet) – while its 4 local subnets pass BACnet traffic. The Automation Controllers use FDR to register with one BASRT-B, but another BASRT-B is needed because its field controllers lack FDR.
Download the full Skorpion EIPR IP Router application guide.