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Generic Routing Encapsulation - GRE

GRE stands for Generic Routing Encapsulation, which is a very simple form of tunneling. With GRE we can easily create a virtual link between routers and allow them to be directly connected, even if they physically aren’t.

1. Network diagram

tunnel-gre

2. Configuration Roadmap

2.1. Basic Configuration

  • R1
interface Loopback0
 ip address 1.1.1.1 255.255.255.255
 ip ospf 1 area 0
!
interface Ethernet0/0
 ip address 192.168.1.1 255.255.255.0
!
interface Ethernet0/1
 ip address 172.30.12.1 255.255.255.0
 ip ospf 1 area 0
!
interface Ethernet0/2
 no ip address
 shutdown
!
interface Ethernet0/3
 no ip address
 shutdown
!
router ospf 1
 router-id 1.1.1.1
!
  • R2
interface Ethernet0/0
 ip address 172.30.12.2 255.255.255.0
 ip ospf 1 area 0
!
interface Ethernet0/1
 ip address 172.30.23.2 255.255.255.0
 ip ospf 1 area 0
!
interface Ethernet0/2
 no ip address
 shutdown
!
interface Ethernet0/3
 no ip address
 shutdown
!
router ospf 1
 router-id 2.2.2.2
!
  • R3
interface Ethernet0/0
 ip address 172.30.23.3 255.255.255.0
 ip ospf 1 area 0
!
interface Ethernet0/1
 ip address 172.30.34.3 255.255.255.0
 ip ospf 1 area 0
!
interface Ethernet0/2
 no ip address
 shutdown
!
interface Ethernet0/3
 no ip address
 shutdown
!
router ospf 1
 router-id 3.3.3.3
!
  • R4
interface Loopback0
 ip address 4.4.4.4 255.255.255.0
 ip ospf 1 area 0
!
interface Ethernet0/0
 ip address 192.168.2.1 255.255.255.0
 shutdown
!
interface Ethernet0/1
 ip address 172.30.34.4 255.255.255.0
 ip ospf 1 area 0
!
interface Ethernet0/2
 no ip address
 shutdown
!
interface Ethernet0/3
 no ip address
 shutdown
!
router ospf 1
 router-id 4.4.4.4
!

2.2 GRE tunnel configuration

  • R1
interface Tunnel100
 ip address 10.100.100.1 255.255.255.252
 tunnel source 1.1.1.1
 tunnel destination 4.4.4.4
!
ip route 192.168.2.0 255.255.255.0 10.100.100.2
  • R2
interface Tunnel100
 ip address 10.100.100.2 255.255.255.252
 tunnel source 4.4.4.4
 tunnel destination 1.1.1.1
!
ip route 192.168.1.0 255.255.255.0 10.100.100.1

Now let try ping between 2 VPC:

image

Read more here