Get Some Tunnel Vision!
Pull back the curtain, turn on the light, put on your x-ray vision goggles and inspect the traffic passing through your cleartext tunnels (GRE, non-encrypted IPSec, or GTP-U tunnels). You want tunnel vision!
You can’t protect against things you can’t see, including sessions tunneled through the firewall. If sessions are tunneled in a protocol such as GRE, ...