#81 Response to Who-Is always sent as Local Broadcast in the I-Am response

[Prasad Kulkarni]

Hi team,

I am using BACnet stack Version 1.3.1 for BACnet protocol revision 19 implementations.

In that, it is observed that the stack responses with an I-Am using the destination address as 192.168.1.255 (local broadcast) when responding to a Who-Is request directed to the device.

Is this OK or there are any changes being made that it will send the I-Am as a directed response.

Thank you!

[Steve Karg]

There are two basic service handlers for the Who-Is service request. One for broadcast and one for unicast. Please configure the handler to your device requirements.

    apdu_set_unconfirmed_handler(
        SERVICE_UNCONFIRMED_WHO_IS, handler_who_is);

or

    apdu_set_unconfirmed_handler(
        SERVICE_UNCONFIRMED_WHO_IS, handler_who_is_unicast);

[Prasad Kulkarni]

Thanks for the reply Steve!

Does that mean it is OK to reply Unicast to a Who-Is which has come from a specific host?

And the only first I-Am to be sent as broadcast and rest we can handle by Unicast?

[Steve Karg]

For devices that are protocol-revision 8 or higher, it is allowed to reply I-Am with unicast.

Sending I-Am as a broadcast at power up and when device ID changes is great for diagnostics (I use it), but sometimes bad for large sites when power fails, then returns, and floods a network with broadcast I-Am as devices power up.

Note: there are some site integrations and devices that incorrectly rely on BACnet I-Am broadcast, particularly in their BACnet/IP setup.

[Prasad Kulkarni]

Thanks Steve.