0Tunneling 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.
R1(config)# interface Tunnel1
R1(config-if)# ip address 172.16.1.1 255.255.255.0
R1(config-if)# ip mtu 1400
R1(config-if)# ip tcp adjust-mss 1360
R1(config-if)# tunnel source 1.1.1.1
R1(config-if)# tunnel destination 2.2.2.2
R2(config)# interface Tunnel1
R2(config-if)# ip address 172.16.1.2 255.255.255.0
R2(config-if)# ip mtu 1400
R2(config-if)# ip tcp adjust-mss 1360
R2(config-if)# tunnel source 2.2.2.2
R2(config-if)# tunnel destination 1.1.1.1
Since GRE is an encapsulating protocol, we adjust the maximum transfer unit (mtu) to 1400 bytes and maximum segment size (mss) to 1360 bytes. Because most transport MTUs are 1500 bytes and we have an added overhead because of GRE, we must reduce the MTU to account for the extra overhead. A setting of 1400 is a common practice and will ensure unnecessary packet fragmentation is kept to a minimum.
After configuring tunnel, two tunnel endpoints can see each other can verify using an icmp echo from one end.
R1# ping 172.16.1.2
