What are GRE tunnels and how are they utilized?
GRE tunnels are often times used to encapsulate IP within IP. They have many uses (without security, this is not a vpn) across an open network. Tunnel interfaces on a Cisco router have two basic parts. • The destination ip address of the other end of the tunnel. This is reachable IP of the other router you want to tunnel to. • The local ip address of your Tunnel interface itself. Once the tunnel is established, this local ip address is basically directly connect to local address of the other end of the tunnel. This could be a privately addressed /30 point-to-point subnet. Yes, it’s really as simple as this… interface Tunnel1 tunnel destination 4.2.2.2 ip address 10.1.1.1 255.255.255.252 Some examples.. 1. Say you had a multicast stream at the office you wanted to watch at home, you could build a GRE tunnel between your home router and your office router and enable PIM on the tunnel interface to router the multicast to your house. Normally multicast wouldn’t route across the internet,
