The BACnet discovery uses two services – called ‘Who-Is’ and ‘I-Am’. They rely on the use of broadcasts.
Routers join IP networks together so messages from one network can be sent to another. Most routers do not forward broadcast messages and this means discovery cant discover devices on another network.
To solve this problem BACnet provides a technology called BBMD – BACnet/IP Broadcast Management Device.
In overview the technology is simple. You install a BBMD (might be a physical device or just a software application on a computer) on each network. You can configure the BBMD by specifying the IP Address and mask of the each BBMD. This makes both BBMD configs identical. When the one BBM receives a broadcast, it forwards the messages to the other BBMD which in re-broadcasts on the other network. They are configured by BDT files and these may be modified on the fly using selected Bacnet services.
The technology also provides for foreign device registration. This allows a device on one network to communicate with a device on another network by using the BBMD to forward and route the messages.