Configure a GRE Tunnel

Configuration Condition

Before configuring a GRE tunnel, ensure that:

Interface IP addresses have been configured so that neighbor nodes are reachable at the network layer.
Tunnel interfaces have been created and the basic parameters have been configured. (Refer to tunnel interface configuration manual.)
A unicast protocol has been configured so that the routes at the two ends of the tunnel are reachable.

Table 10-2 Configuring a GRE tunnel

Step	Command	Description
Enter the global configuration mode.	configure terminal	-
Enter the tunnel interface configuration mode.	interface tunnel tunnel-number	-
Configure the tunnel interface address	ip address ip-address { mask \| mask-length }	Mandatory By default, no address is configured for the tunnel interface.
Configure the tunnel interface mode to GRE over IPv4.	tunnel mode gre ip	Optional. By default, the tunnel interface mode is GRE over IPv4.
Configure the source address or interface name on the tunnel interface.	tunnel source { ip-address \| interface-name }	Mandatory. By default, the source address and interface name are not configured on the tunnel interface.
Configure the destination address or host name on the tunnel interface.	tunnel destination { ip-address \| hostname }	Mandatory. By default, the destination address and host name are not configured on the tunnel interface.

On the two ends of a tunnel, the source addresses and destination addresses must be configured. The source address of one end is the destination end of the other end.
In configuring the source of a tunnel, if the interface mode is adopted, the primary address of the source interface is used as the source address of the tunnel.
The two ends of a tunnel must be configured with the same tunnel mode; otherwise, transmission through the tunnel fails.
On one device, two or more tunnels with the same tunnel mode, source address and destination address are not allowed.