Auto discovery on a separate subnet (as foreign device)
When running the discovery tool from a subnet separate from the BBMD and BACnet devices, it seems like the auto discovery tool is seem as a ‘foreign device’, so the devices do not respond correctly. There is no BBMD on the subnet the connector is on.
When analyzing a Wireshark trace, the discovery tool sends an ‘Original -Broadcast-NPDU’ Who-Is request to the BBMD on the other network, because it is seen as a foreign device, the request is not forwarded onto the sub-devices by the BBMD on this separate subnet. We can see that no I-am requests are sent back to the discovery tool.
As the discovery tool is seen as a foreign device, we believe it will need to use a Distribute-Broadcast-To-Network type of Who-Is request to get the BBMD on the other subnet to perform the broadcast and return the results to the tool. (Yet Another BACnet Explorer was able to discover the devices using DBTN)
The functionality for the connector to be able to use this type of broadcast would be good for similar architectures, the ability for it to either automatically try the DBTN broadcast, or for the user to manually force the broadcast would be very helpful.