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

Aleksander Morgado aleksander at
Wed Mar 10 09:01:03 UTC 2021


> > 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
> >

In the effort of obfuscating the info, you rewrote a key part of that
APN string making it very difficult to understand :)

That "bla.bla" at the end refers to the MNC and the MCC of the
network, and when a context APN is shown like that, you can assume
that's the initial EPS bearer that's already connected as soon as
you're registered.
So, the modem registers in the network, you run CGDCONT? and you get
CID=1 reported with APN "". That means
your modem is registered in the network with MCC 002 and MNC 001 and
If you attempt to now launch a connection with APN "",
ModemManager will lookup an exact APN match, and for the purposes of
APN matching, "" matches the APN
"", so that's why CID=1 is used while in LTE.

> > The APN to use is actually but MM reports:
> >
> > <debug> [1613584864.390514] [modem0/bearer0] found 3 PDP contexts
> > <debug> [1613584864.391031] [modem0/bearer0]   PDP context [cid=1]
> > [type='ipv4'] [apn='']
> > <debug> [1613584864.391381] [modem0/bearer0]   PDP context [cid=2]
> > [type='ipv4'] [apn='']
> > <debug> [1613584864.391730] [modem0/bearer0]   PDP context [cid=3]
> > [type='ipv4'] [apn='']
> > <debug> [1613584864.409742] [modem0/bearer0] looking for best CID
> > matching APN '' 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).

That is what happened while in LTE, yes.

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

I don't understand this statement Giacinto. While in 2G/3G, the logs
from Norbert show that running CGDCONT? show no APN already connected,
and CID=1 reports the empty APN and not the initial EPS bearer APN as
the modem isn't registered in LTE. The CID lookup done by ModemManager
ends up finding the context with CID=3 and APN "" and it
tries to connect that one, only that one, no other one. Why would it
fail like that?


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