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I have a Rasberry Pi with two network interfaces.

One is a ppp interface connecting to a cellular modem over USB.

The other is a local ethernet connection with an ethernet cable connected to a local device.

The ppp interface works well when I start a systemd service to connect to it on boot-up.

However, as soon as I plug in the ethernet cable and boot the device, the eth0 interface and ppp both show up on ifconfig, but with the ppp interface I can not connect out to anything, for example ping 8.8.8.8 states that the destination is unreachable. If I plug out the ethernet cable tho again, it works fine.

Any idea what is going wrong here and why the two connections are not working at the same time? Thanks

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Reset to default
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When the ethernet cable is connected the device is getting a new default route which is being preferred, but is possibly invalid. You should be able to see/manipulate this with the route or ip route command.

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  • Yes that's the problem. I can fix it with route add default gateway "address" . However this needs to be solved on startup automatically. No matter what I do. If I run the script from rc.local, .bashrc, insde rc5.d ,whatever, it never works for me unless i enter the command myself normally on the terminal after i log in
    – Engineer999
    Feb 3, 2019 at 6:16

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