Discover BACnet device in different network with bacnet4j

Question

I am using bacnet4j library to discover Remote BACnet devices. In the same network, everything is working as expected. But I could not find the BACnet controller device located in the different network.

String hostAddress="10.8.102.28";
IpNetwork network = new IpNetworkBuilder().withPort(47808).withSubnet(hostAddress, 24).build();
Transport transport = new DefaultTransport(network);
LocalDevice localDevice = new LocalDevice(Integer.decode(this.settings.getDeviceId()), transport);
localDevice.initialize();

//Finding remote device
int remoteId=1234;
RemoteDeviceFuture remoteFuture = RemoteDeviceFinder.findDevice(localDevice, remoteId);
RemoteDevice remoteDevice = null;
try {
        remoteDevice = remoteFuture.get(); //remote device is null here
    } catch (Exception e) {
        LOGGER.error("Remote device with id " + remoteId + " does NOT exist!");
    }

The above code snippet finds remote device with a given remoteId in the same network. But it can not find a device located in different network. Is there anything wrong here?

Answer 1

That is actually by design. The discovery process takes place using broadcast "Who-Is" messages that are not routed via IP routers. So any devices on a different IP subnet are not discovered this way. If you know the IP address of the remote device, you may be able to configure/program the IP directly as a static IP address.

However, there is something called a BBMD (BACnet Broadcast Management Device). One of these needs to be placed on each subnet, each configured with the IP address of the other BBMD. The BBMDs will intercept the BACnet related broadcasts on its subnet, send it across to the peer BBMD on the other subnet, which will retransmit the broadcast, effectively bridging the two (or more) subnets.

OR, your could configure/program your client as a "Foreign Device", have it register with a BBMD on the far subnet, which will achieve much the same.

BBMDs are fairly common. Most (all) BACnet/IP to BACnet MS/TP routers have the functionality. A lot of BACnet devices also allow the functionality to be enabled.

Hope this helps. See http://www.bacnetwiki.com for more.

Answer 2

UDP/IP Broadcasts - that are used as part of the 'Who-Is' (BACnet) service, generally are not routed by default and in most cases will not be allowed to be routed - e.g. security concerns been the main reason.

But if routing is in place, you could send unicast/directed traffic to the device in question.

(If I remember correctly, theoretically you should now be able to now send a Who-Is as a unicast/directed request - but even if I'm correct in saying that, it's highly likely that the majority of devices will only be listening for Who-Is services via broadcasts only.)

BBMD's are not strictly necessary - and have been considered as security concern as they can give out too much info.

It can be possible to use (effectively) bog-standard network routing instead of a BBMD - having traffic fly across a few different/target VLANs.

(Also be aware of mixing the use of a private IP(v4) address - of your choosing, and a public IP(v4) address - between the client & server/serving-device, you might encounter issues.)