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Unable to learn on 3rd party device.

Issue

After receiving the CyberStations Who Is broadcast  via the BCX4040 the third party device responds with an I Am packet.  However, the BCX is not passing the information back to Cyberstation. 

Environment

  • Cyberstation
  • BACnet
  • Who is
  • I Am
  • 3rd Party
  • Find Bacnet device
  • Learn

Cause

In the attached mstp capture images, neither of the responses from the Automated Logic device at packets 1713 and 5379 have the necessary information that would cause the bCX or any BACnet router to send the message to another network.

3rd Party device responds with an I Am

Looking at the NPDU layer of either message, you’ll notice a destination network (DNET) is not indicated.

If you compare these messages with those from the b3 (at packet 1716 or 5382), you’ll see a DNET of 1 indicated, which is why the I-Am’s from the b3 reach Cyber, as Cyber belongs to Network 1.
BACnet end devices are expected to dynamically detect the presence of SNET/SADDR information in the NPDU and reply with a message that includes the matching values for the DNET/DADDR.

b3 responds with an I Am

In the preceding Who-is packets you can see that the network number has been communicated to the third party device

Resolution

BACnet routers or BACnet Building Controllers do not cache or otherwise correlate requests and responses.

BACnet end devices are expected to dynamically detect the presence of SNET/SADDR information in the NPDU and reply with a message that includes the matching values for the DNET/DADDR.

The role of the BACnet router is to remove the DNET/DADDR information and add the SNET/SADDR information when they route messages from one network to another.


 

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Revision #:
2 of 2
Last update:
‎2018-09-10 11:24 PM
Updated by:
 
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