Generic Routing Encapsulation (GRE) is a network tunneling protocol developed by Cisco Systems that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links or point-to-multipoint links over an Internet Protocol network.
When we use GRE?
GRE is used when packets need to be sent from one network to another over the internet.
Generic Routing Encapsulation (GRE) has some disadvantages:-
GRE tunnel is an encapsulation protocol and does not perform any encryption.
When we create a GRE point-to-point tunnel without any encryption is extremely risky as sensitive data can easily be extracted from the tunnel and misused by others. ( That’s the reason we used IPSec to add an encryption layer and secure the GRE tunnel with the help of IPSec we get army-level encryption).
GRE tunnel uses a ‘tunnel’ interface a logical interface configured on the router with an IP address where packets are encapsulated and decapsulated as they enter or exit the GRE tunnel.
All tunnel interfaces of participated routers must always be configured with an IP address that is not used anywhere else in the network.Each tunnel interface is assigned an IP address within the same network as other Tunnel interfaces. Before you configure you must adjust (MTU) maximum transfer unit and MSS maximum segment size.
Let's see the configuration:-
Topology_
Goal:
- configure the topology as per the diagram.
- assign IP addresses respectivly to their interfaces as per the topology.
- configure default and static routing.
- configure point - to - point tunnels between router 1 to 3 and router 1 to router 4
R1(config)#interface serial 4/0
R1(config-if)#ip address 1.1.1.1 255.0.0.0
R1(config-if)#no shutdown
R1(config-if)#interface fastethernet 0/0
R1(config-if)#ip address 10.1.1.1 255.0.0.0
R1(config-if)#no shutdown
R1(config-if)#no keepalive
R1(config-if)#exit
R2(config)#interface serial 4/0
R2(config-if)#ip address 1.1.1.2 255.0.0.0
R2(config-if)#no shutdown
R2(config-if)#interface serial 4/1
R2(config-if)#ip address 3.3.3.1 255.0.0.0
R2(config-if)#no shutdown
R2(config-if)#interface serial 4/2
R2(config-if)#ip address 4.4.4.1 255.0.0.0
R2(config-if)#no shutdown
R2(config-if)#interface fastethernet 0/0
R2(config-if)#ip address 20.1.1.1 255.0.0.0
R2(config-if)#no shutdown
R2(config-if)#no keepalive
R2(config-if)#exit
R1#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 10.1.1.1 YES manual up up
Serial4/0 1.1.1.1 YES manual up up
R4#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 40.1.1.1 YES manual up up
Serial4/2 4.4.4.2 YES manual up up
R1(config)#ip route 0.0.0.0 0.0.0.0 serial 4/0
R1(config)#exit
R2(config)#ip route 1.0.0.0 255.0.0.0 serial 4/0
R2(config)#ip route 10.0.0.0 255.0.0.0 serial 4/0
R2(config)#ip route 30.0.0.0 255.0.0.0 serial 4/1
R2(config)#ip route 3.0.0.0 255.0.0.0 serial 4/1
R2(config)#ip route 4.0.0.0 255.0.0.0 serial 4/2
R2(config)#ip route 40.0.0.0 255.0.0.0 serial 4/2
R2(config)#exit
R4(config)#ip route 0.0.0.0 0.0.0.0 serial 4/2
R1#ping 20.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 20.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/33/60 ms
R1#ping 30.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 30.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/38/68 ms
R1#ping 40.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 40.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/23/36 ms
R1(config)#interface tunnel 21
R1(config-if)#ip address 192.168.1.1 255.255.255.0
R1(config-if)#ip mtu 1400
R1(config-if)#ip tcp adjust-mss 1360
R1(config-if)#tunnel source 1.1.1.1
R1(config-if)#tunnel destination 3.3.3.2
R1(config-if)#exit
R1(config)#end
R1(config)#router eigrp 6123
R1(config-router)#network 192.168.1.0
R1(config-router)#network 10.0.0.0
R1(config-router)#no auto-summary
R1(config-router)#exit
R1#ping 192.168.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 24/28/32 ms
R1#show ip interface brief | exclude ?
LINE Regular Expression
R1#show ip interface brief | exclude unass
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 10.1.1.1 YES manual up up
Serial4/0 1.1.1.1 YES manual up up
Tunnel21 192.168.1.1 YES manual up up
R1#traceroute 30.1.1.1
Type escape sequence to abort.
Tracing the route to 30.1.1.1
VRF info: (vrf in name/id, vrf out name/id)
1 192.168.1.2 56 msec 52 msec 20 msec
R1#traceroute 30.1.1.1 source 1.1.1.1
Type escape sequence to abort.
Tracing the route to 30.1.1.1
VRF info: (vrf in name/id, vrf out name/id)
1 192.168.1.2 24 msec 32 msec 24 msec
R1#traceroute 40.1.1.1
Type escape sequence to abort.
Tracing the route to 40.1.1.1
VRF info: (vrf in name/id, vrf out name/id)
1 1.1.1.2 16 msec 16 msec 20 msec
2 4.4.4.2 16 msec 32 msec 40 msec
R1(config)#interface tunnel 41
R1(config-if)#ip address 192.168.40.1 255.255.255.0
R1(config-if)#ip mtu 1400
R1(config-if)#ip tcp adjust-mss 1360
R1(config-if)#tunnel source 1.1.1.1
R1(config-if)#tunnel destination 4.4.4.2
R1(config-if)#exit
R1(config)#end
R1(config)#router eigrp 6123
R1(config-router)#network 192.168.40.0
R1(config-router)#exit
R1(config)#end
R4(config)#router eigrp 6123
R4(config-router)#network 192.168.40.0
R4(config-router)#network 40.0.0.0
R4(config-router)#no auto-summary
R4(config-router)#end
R1#traceroute 40.1.1.1
*Feb 11 12:46:27.511: %SYS-5-CONFIG_I: Configured from console by console
R1#traceroute 40.1.1.1
Type escape sequence to abort.
Tracing the route to 40.1.1.1
VRF info: (vrf in name/id, vrf out name/id)
1 192.168.40.2 20 msec 52 msec 40 msec
R2#traceroute 40.1.1.1 source 20.1.1.1
Type escape sequence to abort.
Tracing the route to 40.1.1.1
VRF info: (vrf in name/id, vrf out name/id)
1 4.4.4.2 28 msec 28 msec 16 msec
R2#ping 30.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 30.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/16/20 ms
R2#ping 40.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 40.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/10/12 ms
R2#ping 10.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/15/28 ms
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