What is a BBMD, and Why Do We Need It?

As BACnet networks get bigger and network infrastructure gets more complicated, BACnet/IP Broadcast Management Devices (BBMD) become necessary. Installers often configure these devices incorrectly despite their importance which creates the conditions for a broadcast storm.

What is a BBMD?

To understand what these are and why we need them, we first need to understand what a broadcast is. Network protocols like BACnet use broadcasts to efficiently communicate public information. A broadcast is sent once and received by all other devices on the same network. As a network gets bigger, broadcasts start to become a problem since they have to be processed by all devices. To address this problem, IT infrastructure network routers that connect different networks together block broadcasts from crossing networks. This presents a problem for us with BACnet because it requires broadcasts for discovery of devices and objects. Broadcasts are how BACnet devices announce that they exist and have points to share.

Each broadcast management device has a list of all others on other networks in its broadcast distribution table. They act as co-conspirators, getting around the broadcast blockade by smuggling the broadcasts into directly sent packages that routers allow to pass through them.

Here’s a point-form description of how they function:

  1. A BBMD packages BACnet broadcast messages that it hears on its own network into an envelope.
  2. It sends these packages directly to BBMDs on the other networks using its distribution table.
  3. Since those packages are sent directly from one device to another, routers won’t block it.
  4. When a BBMD gets a broadcast package from another BBMD, they open up that envelope and rebroadcast the message locally.

This allows your BACnet network to get around any routers in between that would block the broadcast traffic. It’s essentially smuggling, except it’s legal!

Visual Example of Broadcast Communication

Below is an example network architecture with three separate IPv4 networks and three routers. I chose the IP addresses in the example to be similar to what you’d be familiar with at home.

Example Network Architecture

An illustration of an example network. There are three separate subnets, each with their own router.

If one of the devices on the network sends a broadcast message, the other device on that network will see it. That same message will be blocked at the router and reach no other devices. This is standard router practice: they keep broadcasts local.

BACnet Broadcast Message Sent

An illustration of an example network with three separate subnets, each with their own router. In this image, a device on one of the subnets is sending a BACnet "Who-Is" message, which is being blocked at the router. The message can only reach other devices in the same subnet.
When a BBMD receives the broadcast message it encapsulates it and sends it a direct message to every other BBMD in its Broadcast Distribution Table (BDT). These other BBMDs open up the package and rebroadcast the contents locally, permitting BACnet broadcast traffic to permeate the entire network.

BACnet Broadcast Message Encapsulated and Delivered by the BBMD

An illustration showing how a BBMD gets around the broadcast block at the router level by "unicasting" the broadcast messages to other BBMDs. The BBMDs act as co-conspirators, getting the broadcast messages around routers that would block them.Common mistakes with configuring BBMDs

  • Multiple BBMDs configured on the same BACnet network is a huge, common problem. If you have more than one BBMD on a network it will cause a broadcast storm.
  • Using different broadcast distribution tables on different BBMDs.
  • Using the same network number for networks separated by routers.

Further Reading on BBMDs

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