A BBMD, or BACnet broadcast management device, sends a unicast message from one BACnet or IP device on a subnet to other subnets. Once the message gets to the destined subnet, the message is rebroadcast. A duplicate BBMD is where multiple devices on the same network are set as BBMDs, and are rebroadcasting the same messages.

A BBMD, or BACnet broadcast management device, sends a unicast message from one BACnet or IP device on a subnet to other subnets. Once the message gets to the destined subnet, the message is rebroadcast. A duplicate BBMD is where multiple devices on the same network are set as BBMDs, and are rebroadcasting the same messages.

The reason duplicate BBMDs are so dangerous, is they can increase the amount of traffic by double or more. The network gets more traffic, and the BBMD at the other end gets more traffic. This can take down a network entirely, making it inaccessible. Duplicate BBMDs are caught by Visual BACnet in the “Duplicate BBMD” diagnostic check.  It identifies single device IDs receiving the same information from multiple sources.

A duplicate BBMD is normally caused by a setup issue, typically on a mixed vendor site. On a new site, the first vendor sets up multiple networks and connects them with BBMDs. A site upgrade or change takes place, a new vendor wins the contract, and they add devices to each network. In order to connect the devices, they put a BBMD on each subnet. This can continue, resulting in two, three or more BBMDs on each subnet, and double or triple the traffic.There should only be one BBMD per network.

To resolve the problem, start by:

1. Finding all the problem vendors using the IP addresses, vendor identifier, and MAC address in Visual BACnet.
2. Isolating networks if you don’t need integration.
3. If not, assigning one vendor and their BBMDs to the task.

A good rule of thumb is to use one vendor’s BBMD, as it’s easier to make site-wide changes. Learn more in the webinar recording!