GRE Tunnel authentication

I’m looking at the GRE Configuration guide for Cisco 881 and I cannot see a reference to how the tunnel is authenticated. Can anyone advise?

Hi,

GRE tunnels don’t have authentication like IPSec. They rely on both sides knowing the IP of the other, so only specific sources can tunnel the traffic against specific nodes

Thanks @Pablo_Valenzuela - I didn’t appreciate that.

This same example is shown, taught and explained in the Zscaler Cloud Certified Professional course and than hammered home in the Labs, where both IPsec and GRE are configured.