Ubuntu Server Tunnel

auto tun1
iface tun1 inet static
    address {{local-private-ip4-address}}
    netmask 255.255.255.252
    pre-up iptunnel add tun1 mode gre local {{local-public-ip4-address}} remote {{remote-public-ip4-address}} ttl 255
    up ifconfig tun1 multicast
    pointopoint {{remote-private-ip4-address}}
    post-down iptunnel del tun1

If you are running a firewall on your Ubuntu server, you will need to make sure to allow GRE tunnelling through the firewall.

-A PUBLIC -p gre -m comment --comment "GRE Tunneling" -j ACCEPT

Cisco IOS Tunnel

!
interface Tunnel1
 ip address {{local-private-ip4-address}} 255.255.255.252
 ip mtu 1400
 ip tcp adjust-mss 1360
 tunnel source {{local-public-ip4-address || interface-name}}
 tunnel destination {{remote-public-ip4-address}}
!