Converging BACnet Part 2

Following on from Part 1 of the series of converging BACnet networks, Part 2 will look at BACnet Broadcast Management Devices, which allow the BACnet network to be converged. In Part 1 there were four different broadcasts mentioned. Of these four only two would need to traverse the layer 3 boundary.

For the broadcast to traverse the layer 3 boundary a BACnet Broadcast Management Device or BBMD can be used. A BBMD can be a hardware device or a piece of software running a machine on each network. Where these are hardware devices that can standalone or the BBMD may be a function of your controller. As you will see there could be a lot of network traffic that could burden your controller so I would always recommend larger networks having dedicated BBMD devices.

The BBMD operates by taking the layer 2 broadcast frame and encapsulation it into a layer 3 IP packet. This IP packet is then routed across the network where the BBMD on the receiving network reverses this process and the broadcast frame is sent out to all devices.

Every subnet that requires the ability to broadcast messages from another subnet will require a BBMD. Therefore, there must be a BBMD on the subnet where the broadcast originates and a BBMD on the subnet where the message is to be directed.

Within the BBMD there exists a Broadcast Distribution Table or BDT. The BDT lists the IP address of all of the BBMD devices, including themselves, required to build the virtual converged network. There is a limit on the number of entries into a BDT therefore this must be considered while designing out your network.

Where a device, such as an operators workstation, sits on part of the network without a BBMD, these devices can still receive the broadcast through a mechanism called Foreign Device Registration or FDR.

Using the FDR mechanism the BBMD can forward a broadcast message as a unicast message. This mechanism operates as follows. The devices, which would require BACnet broadcasts, sends a request to a BBMD device that supports FDR. The BBMD device will record the details of the controller requesting BACnet broadcasts, into the Foreign Device Table or FDT. The entry into the FDT is stored for a finite amount of time and therefore the device requesting the forwarded broadcast must re-register at regular intervals.

In the final part of this topic, VxLan’s shall be discussed as a way to overcome the limitation of network broadcasts.