John, Combining a BBMD, FD, and router can achieve the goal you after. It just takes some planning and a little more configuration.Message 1 of 5 , Mar 22, 2012View Source
Combining a BBMD, FD, and router can achieve the goal you after. It just takes some planning and a little more configuration.
From: Matsen, Dean C [mailto:dean.matsen@...]
Sent: Wednesday, March 21, 2012 2:32 PM
Subject: [bacnet-ip-wg] RE: Using foreign registration between BBMDs
Last time I investigated this, I came to the conclusion that the behavioral requirements of a BBMD and a registered device are mutually exclusive. As far as I can see, a BBMD can't register as a foreign device with the current specification, for the reasons you stated, and maybe more.
Engineer Software Pr
Alerton Dealer Business
Honeywell Automation & Control Solutions
6670 185th Ave NE
Redmond WA 98052
Phone – 425.897.3980
Fax – 425.869.8445
Suppose that I have BBMD A and Device B on a different subnet. B registers as a foreign device with A. If B sends a PDU with distribute-broadcast-to-network to BBMD A, A will do its thing and forward the PDU according to its BDT and FDT.
If B does not register (or if it allows the registration to time out), and then sends a PDU with distribute-broadcast-to-network to BBMD A, at least some current BBMDs (such as ours) will refuse to forward the PDU, and will return a BVLC-result with an error. (Clause J.4.5 doesn’t explicitly talk about this sort of refusal, but it explicitly does allow a BBMD to return a BVLC-result “if the BBMD is unable to perform the forwarding function”.)
Now suppose that device B is also a BBMD. It wants to register foreign with A, so that it can use a DNS name to find A, and so that their BDTs don’t have to be kept in sync.
When BBMD B needs to forward a local broadcast, it can’t use a distribute-broadcast-to-network message, since that doesn’t contain the source address of the originator of the broadcast.
If BBMD B forwards to BBMD A using forwarded-NPDU, is the PDU likely to be rejected by BBMD A, since B is not a peer BBMD in A’s BDT? Annex J doesn’t say anything explicit, but it does describe A doing a search for B’s address in A’s BDT to see whether or not A needs to do a local broadcast. It is easy to imagine an implementation that A would say “B isn’t in my BDT, therefore it isn’t a BBMD and I will ignore its forwarded-NPDU”.
Am I likely to have this sort of problem? If so, what are people doing to avoid it? I would hate to have to reconfigure all the non-BBMD devices on B’s subnet to register foreign. In addition to configuration hassles, BBMD A would then need to cope with a large number of foreign registrations.
The information contained in this message is privileged and intended only for the recipients named. If the reader is not a representative of the intended recipient, any review, dissemination or copying of this message or the information it contains is prohibited. If you have received this message in error, please immediately notify the sender, and delete the original message and attachments.