The Building Automation industry expects connectivity across a variety of BACnet networks — IP, Twisted-Pair and Ethernet. A BACnet router provides this capability with support for BACnet/IP, BACnet MS/TP and BACnet Ethernet. The ASHRAE 135 standard defines a network layer allowing multiple BACnet networks to send and receive messages over a single BACnet internetwork without concern for different data links. But a BACnet internetwork is different from multiple IP networks defined by subnets. An IP router connects these sub-netted IP networks together while two BACnet/IP networks can be interconnected using a BACnet router. The setup requires configuring both the IP router and BACnet routers appropriately. This configuration across subnets applies equally for access to remote sites across the internet.
Most IP routers will not pass broadcast messages which are crucial to BACnet's operation. The solution is to incorporate BACnet/IP Broadcast Management Device (BBMD) functionality within the BACnet internetwork.
The BBMD concept requires a broadcast message that originates on one subnet be encapsulated into a directed message and sent to all remote subnets since these directed messages will pass through IP routers. Once the encapsulated messages are received on the remote subnets, a BBMD device will decode the message and resend it on its local subnet as a broadcast message.
To learn more, read the Application Note (PDF).