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.
Likewise, the WNC Gateway also includes BBMD capability and it needs its supervisory device’s IP address to be manually configured into its BDT..
If there are multiple WNC Gateways on the same subnet, only one of them needs to be configured into the NAEs BDT (example only 10.10.3.4 and not 10.10.3.5), but each WNC Gateway needs to be configured with the associated NAE’s IP address of the device that holds the BBMD for the subnet that the associated NAE is a member of. In some cases, the subnet’s BBMD will be on the associated NAE, but not always.
Depending on how the Ethernet system is managed on a given site, it’s not always easy to tell if a BBMD is required by looking at the IP address of the device itself. If the Ethernet Default Gateway on the WNC and the Network LAN IP Router Address are different, then the BBMDs need to be configured.
If you are using Static IP (Auto DHCP = OFF), then an IT department assigns these addresses.