You asked, we answered. This time, it’s all about BBMDs.
We’re back with another episode of answering your BACnet questions! The response to our Q&A series has been incredible – thank you to everyone who submitted their questions about OT networks and BACnet. Our Co-Founder and CTO, Ping-Pook Yao, tackled some excellent questions about BBMDs this time around.
Have more BACnet questions? Keep them coming! We love diving deep into these technical topics that help make OT networks more reliable and efficient. Send us a message on LinkedIn, Reddit, or Bluesky, or email us at marketing@optigo.net .
If you missed the video, you can watch it [here]. But for those who prefer to read, we’ve summarized the key points below.
Q1: What exactly is a BBMD?
BBMD stands for BACnet Broadcast Management Device. Think of it as a translator that watches for BACnet broadcast messages, converts them to unicast, and sends them directly to every BBMD in its Broadcast Distribution Table (BDT). When another BBMD receives it, it converts it back to broadcast – pretty clever!
Q2: Why do we need BBMDs anyway?
Here’s the thing: IP routers are designed to block broadcasts across subnets (which is actually great for internet functionality). But BACnet relies heavily on broadcast messages for device discovery, object discovery, alarms, and unconfirmed change of value notifications.
Without BBMDs, if you have a device in one subnet trying to discover devices in another subnet, it simply won’t work. The BBMD essentially bypasses this IP router limitation by converting broadcasts to unicast (which can traverse routers) and then back to broadcast on the other side.
Q3: Do I Always Need a BBMD?
Not necessarily! Whether you need a BBMD – whether you’re running MS/TP only, BACnet/IP only, or a mixed network – comes down to one key factor: do you have multiple subnets?
If all your devices are on a single subnet, you don’t need a BBMD. But as soon as you have multiple subnets, and you can consider the cloud/internet another subnet to reach, you’ll need at least one source and destination BBMD on your OT network.
Q4: BBMD Best Practices
While BBMDs are necessary for most systems, they’re often the Achilles heel of BACnet networks. Here are some best practices to keep in mind:
- Avoid duplicate BBMDs per subnet. When you have multiple vendors, decide which one will handle the BBMD function for each subnet. Multiple BBMDs create duplicate, triplicate, or even more broadcast traffic than necessary.
- Consider split horizon architecture for larger systems. Instead of a full mesh where every BBMD knows about every other BBMD, use a hub-and-spoke model. Often, only the subnets where your servers are located need to see broadcasts from all other subnets.
- Dedicate the right device to be your BBMD when possible. Don’t burden devices that are already handling critical operations like boiler or chiller plant control with BBMD functions. They need their computing resources for their primary tasks.
Troubleshooting BBMDs
BBMDs can be challenging to troubleshoot, and excessive broadcast traffic is one of the primary reasons building automation systems underperform. Tools like Optigo Visual Networks can help you analyze broadcast message volume and identify problematic duplicate BBMDs quickly.
Have more BACnet questions? Send them our way for future episodes! We love diving deep into these technical topics that help make OT networks more reliable and efficient. Send us a message on LinkedIn, Reddit, or Bluesky, or email us at marketing@optigo.net
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Transcript
Question 1
BBMD stands for BACnet broadcast management device. All it does is it watches for BACnet broadcast messages. As soon as it catches one, it converts it into Unicast, and sends it directly to every single BBMD in its BDT. And when the other BBMD receives it, it converts it back from unicast to broadcast.
Question 2:
[BBMDs] exist because in IT an IP router has two primary functions. One is to route from one subnet to another subnet, but one that’s really important-and the reason why our internet works-is that it blocks broadcast across subnets. But that’s not good for BACnet. Device discovery. Object discovery. Alarms. Unconfirmed change of value… all use broadcast messages.
So if you have a device in one subnet and it’s trying to send a discovery to another subnet, if you have an IP route in between without BBMDs, you can never discover across subnets. So a BBMD will collapse, basically, this IP router function that blocks broadcast by forwarding broadcast from one side as a unicast,
which is allowed to traverse IP routers, and it will convert that unicast back to broadcast.
Question 3
So whether you haveMS/TP only network, or only a BACnet/IP network, do you need a BBMD? It really just comes down to whether or not you have multiple subnets. So let’s say you have a BACnet system with only MS/TP. But You have one supervisor, you have one BMS server and it communicates with all the MS/TP devices through BACnet routers And those global controllers, those panels, those BACnet routers are on different subnets. Then you will likely need a BBMD.
If you only have BACnet/IP? Same thing. If your BACnet/IP devices are only on one subnet, then you do not need BBMD. But as soon as there are multiple subnets, you likely need a BBMD.
Question 4
BBMD is a necessary device in most systems. It is also often the Achilles heel of BACnet systems. But there is a few best practices that we should all use.
Number one, avoid having duplicate BBMDs per subnet. Especially in a case where you have multiple vendors, decide which vendor will have a BBMD for that subnet Instead of having multiple BBMDs when you have multiple BBMDs, it duplicates that broadcast and it can duplicate, triplicate and create manifold more broadcast traffic than necessary.
The second is, especially in a system where it’s a little larger, a thousand devices, maybe you have five-plus subnets. And I know many of you out there have 50-plus subnets. Consider using a split horizon, instead of a full mesh. That is, not every BBMD needs to know about every other BBMD. Perhaps only a small group of subnets where your server is needs to see the broadcast messages from all the other subnets. In that case, don’t do a full mesh, use a hub-and-spoke also what we call split horizon BBMD architecture.
One suggestion, if you have the ability to, is to dedicate a device to be a BBMD. And definitely don’t put the BBBM function on the device that’s already very heavy. If that device is already operating a boiler, your chiller plant, don’t use that devices as your BBMD, because now it’s going to have a lot of compute that it has to do for its own programing, in its own sequence, in combination with being able to afford these broadcast messages.
BBMDs are quite difficult to troubleshoot. Excessive broadcast is one of the primary reasons why building automation systems don’t operate at the desired performance.
So, if I may plug in here, use software like OptigoVN to analyze how many BACnet broadcast messages you have, and also where those messages are coming from. Perhaps you have a duplicate BBMD and you can quickly identify those duplications and be able to eliminate it with the use of Optigo Visual Networks.
