Encapsulation – VPN, GRE Tunnel ?

Why do we need GRE Tunnel

GRE Tunnel, it Encapsulate the packet which is quite similiar with VPN IPSEC.

GRE Tunnel can support Dynamic Routing protocol through the tunnel, it is the reason why it takes an advantages. In DMVPN, GRE will play its role.

Another advantage of GRE Tunnel, is easiler to be configured.

CONFIGURATION

Between R1-R2-R3 both has been configured Point to Point ( With EIGRP Protocol)

on R1 & R3, has created Tunnel and Look back interface ( GRE Tunnel + OSPF )

Configuration - GRE
interface Tunnel 0 
Ip add x.x.x.x x.x.x.x
ip mtu 1400
ip tcp adj-mss 1360
tunnel source x.x.x.x
tunnel destination x.x.x.x

After configuration on All Routers. Here as below result on Show interface and IP route

In IP Routing, we can see on R2, only do advertising EIGRP but didn’t learn OSPF.

R1 & R3 do learning EIGRP and OSPF. As between R1 & R3 we have created a GRE Tunnel . OSPF is advertising through the tunnel.

RESULT

Figure below, “OSPF” & “EIGRP”. We can see that OSPF multicast packet, it is under Generic Routing Encapsulation, but EIGRP which is a normal packet transport.

ICMP From 192.168.2.2 to 1.1, it will be via GI interface – EIGRP

ICPM From 172.17.0.100 to 16.0.100, it will be via Tunnel interface – OSPF