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.
Configuration
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 10.1.1.1 255.255.255.252 R2(config)# ip address 10.1.1.2 255.255.255.252 R1(config)# tunnel source 209.165.201.1 R2(config)# tunnel source 198.133.219.87 R1(config)# tunnel destination 198.133.219.87 R2(config)# tunnel destination 209.165.201.1 R1(config)# router ospf 1 R2(config)# router ospf 1 R1(config-router)# network 10.1.1.1 0.0.0.3 area 0 R2(config-router)# network 10.1.1.1 0.0.0.3 area 0
Troubleshooting
show ip interface brief | include Tunnel show interface Tunnel 0