GRE - Generic Routing Encapsulation
GRE - is a tunneling protocol, works on Linux and Cisco, it provides no security. Security can be added with IpSec.
- GRE keepalives use tunnel source and destination addresses not the GRE tunnel IP
- Important when using gre with vrf as src/dest
- 3G backup on router. The 3G link only provides a single IP on provider network, to route subnets, setup a GRE tunnel with the 3G ip as source to a central router.