Maintaining connectivity between multiple devices

[Kasper Holtze]

Hello,

Sorry if this is the wrong place to ask. I'm working on a solution which 
has multiple interfaces for Internet connection. Two QMI modems 
(MC7304), a LAN interface and a WiFi interface. The questions is 
primarily regarding the modems.

Each modems has two SIM cards, and I've build a small piece of software 
to switch between these, and all of the interfaces. All the switching is 
working fine. I am switching between modems/interfaces by 
opening/closing the interfaces (ifup/ifdown), and that way making sure 
the right interface is in use. Only the interface which "should be used 
right now", is active.

However, I am seeing issues getting connectivity through the modems - 
sometimes I cannot get an IP address from the modem, even though I can 
see that the modem is connected and has an address.

I have recently switched from 802-3 to raw-ip, by upgrading Linux from 
4.4 to 4.9. That seems to work fine, and /sys/class/net/wwanX/qmi/raw_ip 
is always set to Y (is echo'ing Y to this the right way to go?).

I configured the modems to use autoconnect, so that even if there's 
issues with the software, the modems itselfes will connect to the internet.

So, in the hunt of the most stable solution, my questions are:

- I'm wondering if the approach is the best, or if I should not 
open/close the interfaces all the time, and just switch the default 
gateway? Or using libqmi connect functions, and disable autoconnect...

- If I just connect the modems at boot, using autoconnect, will the 
modems reconnect if they are out of coverage for a while? Will the DHCP 
client automatically fetch the new IP address when the connection is 
restores? What if it moves cross-border, will it restore connectivity 
automatically? I guess I could just test this, but it would be nice to 
know some theory about it.

Thank you very much!

Kasper