Appendix: What Does A Bbmd Do - Cimetrics B3075 User Manual

Appendix: What does a BBMD do?

BBMDs forward BACnet broadcast messages within BACnet/IP networks that span multiple IP
subnetworks. The BACnet protocol utilizes broadcast messages for certain functions, such as the
discovery of BACnet devices and for time synchronization. Also, BACnet networks and BACnet routers are
discovered using BACnet broadcast messages. See the
BACnet broadcast messages.
IP routers (also referred to as "IP gateways") join IP networks together so messages from devices
connected to one network can be sent to devices connected to another network. If your BACnet devices'
networks are interconnected via IP routers, then BACnet/IP broadcast messages that are transmitted as
UDP broadcasts will normally be blocked by an IP router. To solve this problem the BACnet Standard
defines the behavior of the BBMD (BACnet/IP Broadcast Management Device) in Annex J.
BBMDs typically use UDP unicast messages to forward BACnet/IP broadcast messages. A BBMD directly
forwards a BACnet broadcast message initiated by a BACnet/IP device on the BBMD's subnet to other
BBMDs and to its registered Foreign Devices in accordance with its configuration. Upon arrival at a
destination BBMD, the message is then transmitted (as a UDP broadcast) on that subnet by the
destination BBMD and is also sent to the destination BBMD's registered Foreign Devices.
A BACnet/IP device can register with a BBMD as a Foreign Device if that configuration option is enabled. A
registered Foreign Device becomes a member of the BACnet/IP broadcast domain, and it will receive
forwarded BACnet/IP broadcast messages from the BBMD. A Foreign Device can also request that
messages be broadcasted by the BBMD on the Foreign Device's behalf. Foreign Devices are typically
connected to an IP subnetwork that does not have a connected BBMD.
Here are some other important characteristics of BBMDs that should be noted.
1. BBMDs only forward BACnet/IP broadcast messages, not unicast messages. BACnet/IP unicast
messages are sent directly by the originating device to the destination device.
2. There is no standard method for a BBMD to discover other BBMDs that it should communicate
with, so each BBMD must be configured with a list of the addresses of the other BBMDs to which it
should forward BACnet/IP broadcast messages.
3. A BBMD that receives a forwarded BACnet/IP broadcast message from another BBMD will not
forward that message to any other BBMDs.
4. To ensure that BACnet/IP broadcast messages flow in both directions between two BBMDs and
between devices connected to their respective IP subnets, each BBMD must be configured to
forward broadcasts to the other BBMD.
5. In almost all situations, there should be at most one BBMD attached to an IP subnetwork
containing BACnet/IP devices.
To find out more about BACnet broadcast management and how to build a wide area network with BACnet
you may refer to the classic article "Building Wide-Area Networks with BACnet" written by Bill Swan.
BACnet Standard for more information about
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