GRE is a tunneling protocol that encapsulates packets inside IP to transport traffic across an IP network. The document describes configuring GRE tunnels between routers R1 and R2 to connect their private networks and exchange routing information. This creates a virtual private link between R1 and R2 to pass traffic even though they are connected through the public internet. Interface configurations on R1, R2 and the internet router are provided to establish the GRE tunnels and associated routing.

1. GRE Tunnel Configuration
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3. GRE (Generic Routing encapsulation)
 It is the simplest protocol
 It encapsulate inside or outside ip address
 GRE is nothing to do with encryption that IPsec provide
 We use GRE for Multicast traffic
 It allow us to start treating the VPN connection like we using private link

4. R1 interface Configuration
 R1(config)#interface serial 1/0
 R1(config-if)#description CONNECTED WITH INTERNET
 R1(config-if)#ip address 100.100.100.2 255.255.255.252
 R1(config-if)#no shut
 R1(config-if)#exit
 R1(config)#interface fastEthernet 0/0
 R1(config-if)#description CONNECTED WITH SW1
 R1(config-if)#ip address 192.168.2.1 255.255.255.0
 R1(config-if)#no shut
 R1(config-if)#exit

5. Internet Router interface Configuration
 internet(config)#interface serial 1/0
 internet(config-if)#description CONNECTED WITH R1
 internet(config-if)#ip address 100.100.100.1 255.255.255.252
 internet(config-if)#clock rate 64000
 internet(config-if)#no shut
 internet(config-if)#exit
 internet(config)#interface serial 1/1
 internet(config-if)#description CONNECTED WITH R2
 internet(config-if)#ip address 200.200.200.1 255.255.255.252
 internet(config-if)#clock rate 64000
 internet(config-if)#no shut
 internet(config-if)#exit

6. Router 2 interface Configuration
 R2(config)#interface serial 1/1
 R2(config-if)#description CONNECTED WITH INTERNET
 R2(config-if)#ip address 200.200.200.2 255.255.255.252
 R2(config-if)#no shut
 R2(config-if)#exit
 R2(config)#interface fastEthernet 0/0
 R2(config-if)#description CONNECTED WITH SW2
 R2(config-if)#ip address 10.250.250.1 255.255.255.0
 R2(config-if)#no shut
 R2(config-if)#exit

7. GRE Tunnel interface Configuration in R1
and R2
 R1(config)#interface tunnel 1 (1:- Tunnel Number)
 R1(config-if)#description TUNNEL DIRECTLY CONNECTING TO R2
 R1(config-if)#tunnel source 100.100.100.2 (source Public ip address of R1)
 R1(config-if)#tunnel destination 200.200.200.2 (Destination Public ip address of
R2)
 R1(config-if)#ip address 172.16.16.1 255.255.255.252 (Private ip address for
Tunnel)
 R1(config-if)#exit

8.  R2(config)#interface tunnel 2 (2:- Tunnel Number)
 R2(config-if)#description TUNNEL DIRECTLY CONNECTING TO R1
 R2(config-if)#tunnel source 200.200.200.2 (source Public ip address of R2)
 R2(config-if)#tunnel destination 100.100.100.2 (Destination Public ip address of
R1)
 R2(config-if)#ip address 172.16.16.2 255.255.255.252 (Private ip address for
Tunnel)
 R2(config-if)#exit

9. Protocol Configuration in R1 and R2 to
exchange their route
 R1(config)#router rip
 R1(config-router)#version 2
 R1(config-router)#network 192.168.2.0 {R1 internal network}
 R1(config-router)#network 172.16.16.0 {R1 Tunnel Network}
 R1(config-router)#no auto-summary
 R1(config-router)#exit
 Ø R1(config)#ip route 0.0.0.0 0.0.0.0 s1/0 {default route to internet}

10.  R2(config)#router rip
 R2(config-router)#version 2
 R2(config-router)#network 10.250.250.0 {R2 internal network}
 R2(config-router)#network 172.16.16.0 {R2 Tunnel Network}
 R2(config-router)#no auto-summary
 R2(config-router)#exit
 R2(config)#ip route 0.0.0.0 0.0.0.0 s1/1 {default route to internet}

11. Troubleshooting & Verification Commands
 Show ip interface brief
 Show ip route
 Show interfaces tunnel 2 (Tunnel Number)
Verify using
 Traceroute from PC1 to PC2
 All the traffic will go through Tunnel
GRE tunnel acts as all the devices are connected in same Ethernet
segment And also It hides the public address