Noob with routing protocols - creating a point-to-point link for running a routing protocol with the least implications for packet fragmentation?

Hi,

I've been asked to think about/plan solutions for mobile backup connections for fiber connections. Routing normally works with BGP to the CPE over the fiber. Now I'm wondering what would be the preferred way of creating a point-to-point link over a public mobile network for a backup connection for a backup BGP session? Short and maybe poor description I know. If a tunneling protocol would be needed I was thinking GRE might be what you'd want to use - but on the other hand you probably want some security/authentication as well.

Still I wonder what the implications are for packet fragmentation since I think all kinds of tunneling involves some overhead? My preferred solution would be to use private mobile networks and not use tunneling protocols at all.

If anyone has designed such backup links would be interested to hear how they planned it.