Cisco GRE Tunnel

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Generic Routing Encapsulation (GRE) - RFC 2784 - developed by Cisco is non-secure, stateless (no flow control), site-to-site VPN tunneling protocol. The Protocol Type field holds identifier for a passenger protocol (encapsulated L3 OSI protocol) - defined in RFC 1700 as EtherTypes.

Gre tunnel diagram


Gre tunnel
R1(config)# interface Tunnel0                              R2(config)# interface Tunnel0     
R1(config)# tunnel mode gre ip                             R2(config)# tunnel mode gre ip        !tunnel interface is GRE over IP
R1(config)# ip address            R2(config)# ip address   
R1(config)# tunnel source                    R2(config)# tunnel source
R1(config)# tunnel destination              R2(config)# tunnel destination
R1(config)# router ospf 1                                  R2(config)# router ospf 1     
R1(config-router)# network area 0         R2(config-router)# network area 0


show ip interface brief | include Tunnel
show interface Tunnel 0