Tunneling provides a mechanism to transport packets of one protocol within another protocol.
The protocol that is carried is called as the passenger protocol, and the protocol that is used for carrying the passenger protocol is called as the transport protocol.
Generic Routing Encapsulation (GRE) is one of the available tunneling mechanisms which uses IP as the transport protocol and can be used for carrying many different passenger protocols.
The tunnels behave as virtual point-to-point links that have two endpoints identified by the tunnel source and tunnel destination addresses at each endpoint.
Configuring a GRE tunnel involves creating a tunnel interface, which is a logical interface.
Then you must configure the tunnel endpoints for the tunnel interface.
To configure a GRE tunnel, create a tunnel interface by issuing the interface tunnel command from the global configuration mode.
To configure the tunnel source and destination, issue the tunnel source {ip-address | interface-type} and tunnel destination {host-name | ip-address} commands under the interface configuration mode for the tunnel.
For details on GRE tunnel configuration with examples and related information refer to:
Configuring Logical Interfaces
and the
Cisco GRE Technology Support Page
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Vlan sama
Yes, vlans are very popular. They will surely rule the world.