Cisco Router GRE Client Configuration
Tunnel Configuration
The GRE tunnel configuration on a Cisco Router is fairly easy. If you are accepting a default route from us we suggest using a separate route table / VRF for either the tunnel network or your WAN interface. The WAN interface is probably the easiest as the "front door VRF configuration" is fairly common. You can see this in the Gigabit Ethernet 0/0/0 interface.
As you can see in the Tunnel interface configuration we are then anchoring the tunnel to that WAN interface with the tunnel source command. Finally, take note if we are connecting with you over a publically routable address we will use a /31 or /30 under most circumstances.
interface GigabitEthernet0/0/0
description WAN Interface
vrf forwarding WAN_VRF
ip address dhcp
no cdp enable
interface Tunnel0
description Tunnel to Core Transit
tunnel source GigabitEthernet0/0/0
tunnel destination 198.51.100.1
ip address 192.0.2.1 255.255.255.254
ip tcp adjust-mss 1400