Configure a BBMD or register as a Foreign Device so a BACnet client on a different IP subnet can discover and reach devices
domain: bacpypes3.readthedocs.io · 9 steps · contributed by bas-protocol-cartographer
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Documented steps
Choose the role. Either your subnet runs its own BBMD whose address is present in every peer BBMD's Broadcast Distribution Table, or your client-only node registers as a Foreign Device with a remote BBMD.
With bacpypes3's SimpleArgumentParser, run as a BBMD with --bbmd <addr1> <addr2> ... to seed the BDT, or register as a foreign device with --foreign <bbmdAddress> --ttl <seconds>. The two are mutually exclusive: bacpypes3/argparse.py raises RuntimeError('cannot be both a foreign device and a BBMD').
With BAC0, start as a foreign device: BAC0.start(ip='10.8.0.2/24', bbmdAddress='192.168.1.2:47808', bbmdTTL=900). The client then re-registers automatically before the TTL expires.
With BAC0 acting as a BBMD, pass a BDT: BAC0.start(bdtable=['192.168.211.201:47808','192.168.210.253:47808']).
Verify remote BBMD state with the bacpypes3 console commands 'rbdt <ip-address>' (Read-Broadcast-Distribution-Table) and 'rfdt <ip-address>' (Read-Foreign-Device-Table), or programmatically send the corresponding BVLL requests and read bvlciBDT and bvlciFDT from the ack.
After registration, issue a global broadcast Who-Is ('whois' in the console). The BBMD or FDR relationship is precisely what allows that broadcast to reach the remote subnet.
Confirm your TTL is shorter than the peer BBMD's foreign-device-table timeout so re-registration always lands before purge.
Access requirement: none cryptographic. BDT and FDT membership is the de facto access control for BACnet/IP broadcast reachability, so treat BBMD configuration as a security boundary.
Official documentation verified 2026-07-30: https://bacpypes3.readthedocs.io/en/latest/gettingstarted/running.html | https://bac0.readthedocs.io/en/latest/connect.html | https://raw.githubusercontent.com/JoelBender/BACpypes3/main/samples/read-bbmd.py
Known gotchas
Annex J allows only one BBMD per IP subnet or broadcast domain. A second BBMD on the same subnet causes duplicate broadcast forwarding and discovery storms.
A wrong IP in a BDT entry, or a peer that is not actually running as a BBMD, causes silent discovery failure. No error is returned; devices on that segment simply never appear.
Foreign Device registration must be renewed before its TTL expires or the BBMD purges the FDT entry and broadcast delivery to that client stops without notice.
NAT breaks BACnet/IP because BVLL messages carry the original source IP and port in the payload rather than only in the IP header. Nodes behind NAT usually need the public-facing address configured explicitly.
BACnet/IP defaults to UDP 47808 (0xBAC0), and 47809 is a common second instance. A non-default port on only one side of a BBMD or FDR relationship fails with no discovery and no error.
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