When attaching BACnet devices to IP networks, it is possible that the IP network has been sub-netted through the use of IP routers. 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. We have a video, BACnet Traffic Across IP Subnets, that explains how to implement this solution.
The BBMD concept requires that a broadcast message originating 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. Therefore, it would appear that a BBMD device must be present on each subnet in order to provide this encoding and decoding function. However, this is not the case if all the BACnet/IP devices support Foreign Device Registration (FDR). At a minimum, one BBMD device is required to be located on one of the subnets with FDR devices registering to this one BBMD. This is what is shown in the example below with a BASrouter providing BBMD functionality while allowing for foreign device registration. Notice that connecting to a BACnet MS/TP network is an option.
Watch the BACnet Traffic Across IP Subnets video or visit the EIGR IP Router page to learn more.