What a BBMD Is
A BACnet Broadcast Management Device (BBMD) forwards BACnet/IP broadcast messages across IP subnet boundaries. Without BBMDs, Who-Is and I-Am discovery broadcasts stay confined to their local subnet.
Every subnet that contains BACnet/IP devices must have exactly one BBMD (or none if all devices are on a single subnet).
How It Works
Each BBMD maintains a Broadcast Distribution Table (BDT) listing the IP address and broadcast mask of every other BBMD in the network. When a BBMD receives a local broadcast, it forwards it as a directed message to every BBMD in its BDT, which then re-broadcasts it on their local subnets.
When You Need One
| Scenario | BBMD Required? |
|---|---|
| All devices on one subnet | No |
| Devices on 2+ subnets | Yes — one per subnet |
| Single remote device accessing a subnet | FDR may suffice instead |
[!WARNING] Misconfigured BDTs are a top cause of “device not found” issues in multi-subnet BACnet networks. Every BBMD must list every other BBMD — partial tables cause one-way discovery failures.
Chipkin Products
Chipkin offers a standalone BBMD appliance (CAS BACnet BBMD) for networks that lack a native BBMD. QuickServer and FieldServer gateways can also act as BBMDs.