The idea behind BBMDs, or BACnet Broadcast Management Devices, is simple: BACnet devices need a way to broadcast messages across IP sub-networks to discover and communicate with other devices in a larger BACnet network. That’s where BBMDs come in, translating BACnet packets into a format that won’t get blocked by an IP router, and back into a format that BACnet devices need to communicate.
Today, BBMDs can be as simple as a software function integrated into an existing BACnet device. But it’s still worth revisiting how this humble solution came about, what potential issues to look out for when managing BBMDs, and to consider what the future has in store for BBMD communications. Let’s look at what BBMD is.
Why Do BACnet Networks Need BBMDs?
The BACnet protocol uses broadcast messages to communicate. This method of “one-to-all” communication for the WHO-IS and I-AM messages of BACnet is critical to how it discovers other devices and networks, and shares information—not only for device discovery but also for alerts, COV notifications and other important commands
On small, dedicated OT networks—like traditional MS/TP networks, and Ethernet LANs— this approach works fine. You don’t need to know anything about the who, what, or where of any target devices on your network when you start, since everyone will get a copy of your WHO-IS.
If it’s networked, you’re bound to get the answer you need (eventually). It’s simple enough for low-power devices, like those in OT networks, to handle.
But on the other hand, it’s also the most resource-intense form of messaging protocol. As you scale up your network, adding devices and sub-networks, the idea of sending a message to the whole network every time you need something, then having every device have to stop and read it, then decide on a response (or non-response), becomes untenable.
IP Networks Hate Broadcasts
Without going into a lot of detail about the technical reasons OSI Layer 3 devices block Layer 2 protocols like broadcast packets, it’s not very hard to picture why. IP networks range from dozens to potentially hundreds of thousands of networked devices in a domain consisting of multiple subnets. IP routers exist specifically to separate those subnets for management. Consider the implications if networks allowed every device to send and receive global broadcast messages. Everything would grind to a halt in seconds. Subnets would cease to function as the network becomes one big broadcast storm. In short, broadcast messages are limited to the subnet the device is on. Any attempt to forward a broadcast message across an IP router is simply blocked.
So What’s a BACnet Device To Do?
BACnet networks increasingly live within IP-based environments. By now, you’ve probably encountered BACnet over IP (BACnet/IP) hardware that enables OT systems to take advantage of the speed and scale of IP architecture. That said, BACnet devices still need to be able to communicate with other devices on other BACnet subnets, regardless of whether it’s IP, Ethernet, or MS/TP.
But how do you do this when IP routers won’t forward broadcast messages, preventing BACnet devices from finding each other? The answer is a go-between. BBMDs act as a sort of forwarding service from one subnetwork to another, so communications can be broadcast locally. At the most basic level, a BBMD translates BACnet broadcast messages into BACnet unicast messages. Here’s Optigo Co-Founder and CTO, Pook-Ping Yao, with a 2-minute explanation of how it all works:
How Does a BBMD Work?
A BBMD (BBMD1) on one IP subnet receives a broadcast message from one of the devices on its network (network1). All BBMDs have an address book of sorts with the IP addresses of other BBMDs on the network, formally called a Broadcast Distribution Table (BDT), that tells it where to send that message.
So BBMD1 translates its broadcast message into a unicast message and forwards that through an IP router to BBMD2, which then translates it back into a broadcast message, and sends it out locally to network2. That’s it in a nutshell!
Sometimes it’s easy to see in action. Here’s Ping with another illustrated example of how BBMDs can work to share BACnet messages across three subnets on a BACnet/IP network.
Watch Out For Duplicate BBMDs
One of the most common troubleshooting issues in BACnet is duplication. That’s because, unlike IP-based devices, BACnet systems still require manual address assignment. It’s easy to get more than one device associated with the same address (IP/MAC address, device instance number, network number, etc.).
With BBMDs, this can be a big issue. For example, on a new site, the first vendor sets up multiple networks and connects them with BBMDs. Over time, a site upgrade or change takes place. A new vendor wins the contract, adding devices to each network. In order to connect the devices, they simply put their own BBMDs on each subnet. This cycle continues, resulting in two, three, or more BBMDs on each subnet, and double or quadruple or more the broadcast traffic. This can take down a network entirely, making it inaccessible.
Manual troubleshooting for duplicate BBMDs can also be tricky, as the issues can’t always be traced back to one consistent source. It can be extremely difficult and time-consuming, splitting networks to try and identify issues, then isolating networks to ensure no future problems.
Fortunately, Optigo Visual Networks (OptigoVN) can resolve duplicate BBMDs faster than ever. OptigoVN’s suite of diagnostics includes a scan for duplicate BBMDs, alerting you instantly to an issue. With a Site Scope add-on, you’ll be able to pinpoint the addresses of the two offending BBMDs and quickly assign new ones. Problem solved in minutes instead of days.
What’s The Future For BBMDs?
BBMDs have steadily evolved. What was once a dedicated piece of hardware is now just a function that can be enabled within the settings of many different BACnet devices, like BACnet/IP routers and gateways. And as with all things tech, it begs the question, what might be next?
Split Horizon
First introduced by the BACnet group at Cornell University, Split Horizon Broadcast Management is a network technique used to prevent routing loops in broadcast networks by ensuring that broadcast traffic is not propagated back onto the network segment from which it originated. In simpler terms, when a device receives a broadcast message, the split horizon rule prevents it from sending that message back out through the same interface it received it on. This is particularly useful in large, complex networks where broadcast traffic can quickly overwhelm the system, leading to inefficiencies or even network outages. By managing how broadcasts are distributed, split horizon helps maintain network stability and efficiency.
Catchpoint has more details in their deep dive into Split Horizon routing.
BACnet/SC
It’s not always fair to speculate, but one idea that continues to surface has to do with BACnet Secure Connect (BACnet/SC), and its purported ability to eliminate the need for BBMDs altogether. Because BACnet/SC works at the Data Link Layer (Layer 2), it effectively replaces the need for static IP addresses and broadcast translations that BBMDs account for now, as well as the need to keep configurations up to date with constant network changes.
Delta Controls’ deep dive into BACnet/SC explains:
“BACnet/SC is a new BACnet Data Link Layer, providing another way to send BACnet traffic between two networks. Traditional BBMD routing is effective at connecting BACnet devices that reside on separate networks, however, it requires firewall exceptions to enable two-way communication, and the traffic between BBMD devices is completely open and unencrypted. BACnet/SC replaces the need for BBMD devices, thus closing a major security hole that has been present throughout the building automation world for many years.”
It’s an interesting concept: reducing the complexity of network design, eliminating the need for manual addressing, and moving to more rigorously encrypted traffic all sound like great advantages. But so far, BACnet/SC is just that, a concept. Despite years of development, BACnet/SC still largely lacks uptake from end users, while IT protocols including OPC UA or MQTT compete in the same space. It’s worth keeping an eye on how BACnet/SC matures over time.
With OptigoVN, managing and monitoring your OT networks, including BBMDs, just got a whole lot easier. With 28 diagnostics to help you quickly identify and resolve BACnet network issues. It’s a game-changer for anyone looking for real ROI in OT network management for clients, or time and money saving across your facilities.
Check it out and see how it can make a difference in your operations. Create your free account, and get started today.
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FAQ: What is a BACnet Broadcast Management Device (BBMD)?
1. What is a BBMD?
A Broadcast Management Device (BBMD) is a component in a BACnet network that facilitates communication between devices on different subnets. It handles the forwarding of broadcast messages to ensure devices on separate subnets can still communicate efficiently.
2. How does a BBMD work?
A BBMD receives broadcast messages from devices on its own subnet and forwards them to other BBMDs within the network. These BBMDs then transmit the message to their respective subnets, ensuring that all devices across the network can receive the broadcast.
3. What are the benefits of using a BBMD?
- Enhanced Communication: It allows devices on separate subnets to share information, improving coordination and data flow.
- Improved Network Efficiency: Reduces the need for extensive routing and supports efficient communication in large-scale networks.
- Simplified Network Design: Helps manage communication without requiring significant network changes or complex routing setups.
4. Are there any challenges with implementing a BBMD?
BBMDs need to be correctly configured to avoid issues like broadcasting loops or excessive traffic. Ensuring that the BBMDs are properly set up and managed is crucial for maintaining network performance and stability.
5. Can multiple BBMDs exist in a network?
Yes, a network can have multiple BBMDs, but they must be carefully configured to avoid redundant or conflicting broadcasts that could impact network performance.
6. Can OptigoVN help monitor BBMDs?
Optigo Networks’ solutions, such as Optigo Visual Networks, help monitor and manage BACnet networks effectively, including the management and optimization of BBMD configurations to support seamless communication across subnets.
