Default bearer

Ulrich Mohr u.mohr at
Thu Nov 25 14:22:34 UTC 2021

Hi Sven,

I just had the same issue on a Toby-L2 (ublox modem), where the default 
bearer always connects on PDP context #4, and does not allow to 
disconnect this bearer. As in your case, setting up a secondary context 
may fail (depending on network policy, in my case)

The ublox plugin handles this by ignoring the error message from the 
modem and pretends that the disconnect process succeeded. With this 
mechanism, MM manages to set up a connection (this is already in MM main 
line). So the state machine seems capable of handle that case.

I then had to make sure that the already connected bearer is selected 
when it matches the requested APN. I think you may omit this step since 
MM searches for a matching context in ID order, so PDP context #1 will 
be selected first anyway (in opposite to my case, where another context 
may be probed first and get selected).

My implementation is not yet completely reviewed and has to be merged 
back into the main branch of MM. But you can preview the changes here:

Perhaps that may help you on your own implementation,

Best regards,


Am 25.11.2021 um 14:55 schrieb Sven Schwermer:
> Hi,
> How does ModemManager handle default bearers that connect automatically? Specifically, I’m working with a Quectel BG95 that automatically connects on PDP context #1. How would this work best with the ModemManager state machine? I’m not sure if the 3gpp profile manager is compatible because that will always try to disconnect the connected bearer which the modem doesn’t seem to like. It responds with error 30 (plus +CGEV: PDN DEACT1) after AT+CGACT=0,1 followed by AT+CGACT=1,1.
> I have tried setting up a connection on a second PDP context, but that fails as well with error 100.
> Best regards
> Sven

Best regards / Mit freundlichen Grüßen / Salutations distinguées

Ulrich Mohr

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