reuse CID=3 problem for 2G/3G?

[Giacinto Cifelli]

Hi Norbert, Aleksander,

On Wed, Mar 10, 2021 at 12:38 AM Norbert van Bolhuis
<norbert.vanbolhuis at ev-box.com> wrote:
>
> Hi,
>
> We're using a Cinterion ELS61-E R2 modem and ModemManager-1.14.0.
> It looks like we're facing a PDN activation problem which maybe caused
> by ModemManager.
>
> Our modem is happily connected, using 4G and PDP Context ID (CID) 1.
> The CID refers to type=ipv4 and APN=blablabla.com.bla.bla.gprs.
>
> The APN to use is actually blablabla.com but MM reports:
>
> <debug> [1613584864.390514] [modem0/bearer0] found 3 PDP contexts
> <debug> [1613584864.391031] [modem0/bearer0]   PDP context [cid=1]
> [type='ipv4'] [apn='blablabla.com.bla.bla.gprs']
> <debug> [1613584864.391381] [modem0/bearer0]   PDP context [cid=2]
> [type='ipv4'] [apn='']
> <debug> [1613584864.391730] [modem0/bearer0]   PDP context [cid=3]
> [type='ipv4'] [apn='blablabla.com']
> <debug> [1613584864.409742] [modem0/bearer0] looking for best CID
> matching APN 'blablabla.com' and PDP type 'ipv4'...
> <debug> [1613584864.410388] [modem0/bearer0] found exact context at CID 1


In general the UE is allowed to enable an APN only once.
Some modems bridge internally when you use a different CID for attach
and for the data PDN.
AFAIK it is not the case for Intel chipsets (ELS61), and therefore MM
should use CID1 only (because it is used for the attach).

For 3G, obviously it fails: two (activated) CIDs are simply 2
contexts, and must have different APNs.

Best Regards,
Giacinto