Generic Routing Encapsulation (GRE) is a multi-carrier protocol that encapsulates packets with an IP header and enables them to pass through the Internet via a GRE tunnel. A GRE tunnel is a virtual interface in which two routers serve as endpoints. The first router encapsulates the packet and sends it over the Internet to a router at the far end of the GRE tunnel. The second router removes the encapsulation and sends the packet towards its destination.
A GRE tunnel is set up as an IP interface, which allows you to use the GRE tunnel as a routing destination. A GRE tunnel can transport multicast packets, which allows it to work with routing protocols such as RIP and OSPF.
To set up a GRE tunnel, you must create the interface and assign it an IP address, a tunnel source address, and a tunnel destination address. GRE tunnels can be configured as next hops on static routes and policy-based routing next hop lists. Packets can also be routed to GRE tunnels dynamically.
Note:
There may be cases in which the GRE tunnel is not used for routing. In such cases, it may not be necessary to assign an IP address to the tunnel.
The main application for GRE tunneling is to allow packets that use protocols not supported on the Internet, or packets that use private IP addresses that cannot be routed on the Internet, to travel across the Internet. The following are examples of situations in which this can be useful:
Providing multiprotocol local networks over a single-protocol backbone
Providing workarounds for networks containing protocols that have limited hop counts, such as AppleTalk
Connecting discontinuous subnetworks
Enabling virtual private networks (VPNs) over a WAN
You can also configure a GRE tunnel to serve as a backup interface. For information on configuring backup interfaces, see Backup interfaces.
