BBMD vs Foreign Device – BAC-5051E Router

BBMD vs Foreign Device – BAC-5051E Router

[MUSIC] The KMC Conquest BAC-5051E is a multi-port
BACnet router full of integrated features. This compact router is powerful enough for
heavy network traffic when permanently installed, yet small enough to be carried as a technician’s
service tool. In this video, we’re discussing two easily
confused set-ups: BBMD vs. Foreign Devices. After logging in to the BAC-5051E router,
you can find the Configuration options under the Routing tab. Here, under each port, you will find a drop-down
menu with options including BBMD, Foreign Devices and PAD. Let’s take a look at what these settings
mean, and when to use each one. BACnet routers forward special broadcast messages
that are sent by BACnet devices and other BACnet routers. These broadcast messages are used by the devices
and routers mainly to discover one another and the operator work station. If you used more than one BACnet router in
your system, your routers almost always are connected on different IP subnets, or networks. IP routers are used to connect IP, not BACnet,
networks together. These IP routers block all broadcast messages,
including BACnet ones. Though the BACnet routers can forward broadcast
messages, the IP routers do not. This lack of broadcast forwarding by the IP
routers prevents the BACnet routers and devices from discovering routers and devices on other
BACnet networks. There are three ways to configure a BACnet
router to work around this issue: BBMD, Foreign Device and PADding. In all three methods, BACnet routers encapsulate
broadcast messages into a unicast message that the IP routers will forward. The concept is called “tunneling,” and
has been done in IT networking for decades. There are many IP protocols that use a tunneling
method to get from “here” to “there” on different subnets. The most commonly used BACnet tunneling method
involves BACnet routers configured with the BBMD and Foreign Device functions. (Since PADding is rarely done, we will not
discuss it here.) A BBMD, or BACnet Broadcast Management Device,
is a BACnet router that tunnels all broadcast messages it receives to all of the other BBMDs
and Foreign Devices it knows within your system. When a BACnet router receives this broadcast
message, it sends it out on all of its directly-connected networks as a normal BACnet broadcast message,
according to the specified destination. All devices in all your interconnected BACnet
networks can eventually receive all broadcast messages, even from BACnet networks that live
on different IP networks. A Foreign Device is either a BACnet device
or a BACnet router that is temporarily connected to a BBMD, allowing it to access the network
through any internet connection. It has to register with a BBMD on its own
subnetwork to receive forwarded broadcast messages, and it can request that messages
be broadcast by the BBMD on its behalf. By registering with the BBMD, the device becomes
a member of the BACnet IP network. It can talk with any BACnet device directly
without registration, but will only receive broadcasts when successfully registered with
a BBMD. There are plusses and minuses to each configuration. When compared to the Foreign Device, BBMD
does have a more complicated set-up process. Since BBMDs are BACnet routers, all interconnected
BBMDs must use the same BACnet network number. With BBMDs, essentially you are creating a
BACnet network that has only BBMD devices on it. But since it is a BACnet network, it is required
to have its own unique BACnet network number. All BBMDs on this same network need to be
configured with this same unique network number, or they will get confused in trying to route
messages. Since BBMDs tunnel broadcast messages as unicast
messages, they communicate point-to-point with other BBMDs. When configuring a BBMD, you must create a
table of all the other BBMDs you want it to be aware of. This table is usually manually configured,
and must match in all BBMDs you are interconnecting. This takes time and effort, and does open
you up to the possibility of human error. The upside of Foreign Devices is that the
BBMD can be configured to automatically allow any Foreign Device to register with it. Thus, the Foreign Device only needs to be
configured with its BBMD’s IP address – there is no “matching table” involved. The downside to the Foreign Device is that
is has a limited time to be registered, called “Time to Live.” The default time is usually 30 minutes, which
means every 30 minutes the Foreign Device must re-register with the BBMD. If it does not and the Time to Live expires,
the BBMD will no longer send broadcast messages to the Foreign Device. Also, if the BBMD that the Foreign Device
is registered with goes down, the Foreign Device has no way of knowing this. Once the BBMD goes live again, it will not
recognize the Foreign Device. Because of this, the Foreign Device configuration
is only recommended for short term use. Our general rule of thumb when configuring
the BAC-5051E router on a BACnet network is simple. For a permanent connection, use the BBMD method. It may take a bit more time and effort, but
this will pay off with fewer issues down the line. If the connection is temporary, the Foreign
Device configuration should be fine. For more about the BAC-5051E router, please
check out our other videos in this playlist and to learn more about any of the solutions
from the Building Geniuses at KMC, visit us on the web at

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