IP Routers typically block broadcast messages from bridging from one subnet to another. This stops message from flooding a subnet with broadcasts throughout an IP network. However, BACnet systems rely on broadcast message to discover and manage devices on those networks. To deal with this, BACnet has defined a BACnet Broadcast Management Device (BBMD).Figure 1.
When a BBMD receives a BACnet broadcast message from a device on its subnet, it forwards (unicasts) that message directly to another BBMDs in its Broadcast Distribution Table (BDT). Upon arrival at the destination BBMD, it will be rebroadcast on that subnet.
Each BBMD keeps a BDT which contains a list of the IP addresses of all the other BBMDs, including itself, in the network. In a Metasys system, all NAE devices include BBMD capability and their BDT is automatically populated with other NAEs in the system from the Site Director. However, the WNC Gateway(s) needs to be manually added into the Third Party BBMDs BDT attribute of the Site Director's Site Object. See Figure 2.