The document details the configuration of a GRE tunnel between two routers (r1 and r2) to enable connectivity between two private networks (192.168.1.0 and 192.168.2.0). It describes various steps including the creation of tunnel interfaces, setting IP addresses, and adding static routes to allow successful pings between devices on each network. Troubleshooting steps are also included to ensure proper communication through the tunnel.

1. CISCO CCNA
PPP MULTILINK CONFIGURATION
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2. GRE TUNNEL CONFIGURATION
Now I will do small Lab:
The Goal is that PC1 (private Network) be
able to Ping PC2 another Private Network, by
going Via R3 which represent internet. R3 has
no connectivity to LAN 192.168.2.0 and
192.168.1.0 /24

3. GRE TUNNEL CONFIGURATION
Here is R1 has default route to ISP (R3):
R1#show ip route
Codes: C – connected, S – static, I – IGRP, R – RIP, M – mobile, B –
BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2, E – EGP
i – IS-IS, L1 – IS-IS level-1, L2 – IS-IS level-2, ia – IS-IS inter area
* – candidate default, U – per-user static route, o – ODR
P – periodic downloaded static route

4. Gateway of last resort is 20.20.20.2 to network 0.0.0.0
20.0.0.0/30 is subnetted, 1 subnets
C 20.20.20.0 is directly connected, Serial0/0/0
C 192.168.1.0/24 is directly connected, FastEthernet0/0
S* 0.0.0.0/0 [1/0] via 20.20.20.2
R1#
GRE TUNNEL CONFIGURATION

5. GRE TUNNEL CONFIGURATION
Also R2 has a default route to ISP (R3):
R2#show ip route
Codes: C – connected, S – static, I – IGRP, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2, E – EGP
i – IS-IS, L1 – IS-IS level-1, L2 – IS-IS level-2, ia – IS-IS inter area
* – candidate default, U – per-user static route, o – ODR
P – periodic downloaded static route
Gateway of last resort is 30.30.30.1 to network 0.0.0.0
30.0.0.0/30 is subnetted, 1 subnets
C 30.30.30.0 is directly connected, Serial0/0/0
C 192.168.2.0/24 is directly connected, FastEthernet0/0
S* 0.0.0.0/0 [1/0] via 30.30.30.1

6. GRE TUNNEL CONFIGURATION
So right now I should be able to ping from R1 to Public IP address of R2
( Think of R1 for Company 1) and think of R2 for Company 2
R1#ping 30.30.30.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 30.30.30.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/10/13 ms
Now seated at PC1 =192.168.1.1 I cannot ping PC2=192.168.2.1
PC>ping 192.168.2.1

7. GRE TUNNEL CONFIGURATION
Pinging 192.168.2.1 with 32 bytes of data:
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Ping statistics for 192.168.2.1:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),
So Goal is to have a GRE Tunnel Between R1 and R2,
over internet; and PC1 be able to Ping PC2

8. GRE TUNNEL CONFIGURATION
Step 1) Create an Int Tunnel 0 on R1 as follow
R1#
interface Tunnel0
ip address 10.10.10.1 255.255.255.252
tunnel mode gre ip
tunnel source Serial0/0/0
tunnel destination 30.30.30.2
!

9. GRE TUNNEL CONFIGURATION
As we see in above I need to use source (I use my S0/0/0 that is
talking to ISP) and also I need to use the destination, which I need
to use Public IP address of R2 = 30.30.30.2
R1#config t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#int tunnel 0
R1(config-if)#
%LINK-5-CHANGED: Interface Tunnel0, changed state to up

10. GRE TUNNEL CONFIGURATION
R1(config-if)#
R1(config-if)#ip add
R1(config-if)#ip address 10.10.10.1 255.255.255.252
R1(config-if)#tun
R1(config-if)#tunnel ?
destination destination of tunnel
mode tunnel encapsulation method
source source of tunnel packets

11. GRE TUNNEL CONFIGURATION
R1(config-if)#tunnel sou
R1(config-if)#tunnel source ?
Ethernet IEEE 802.3
FastEthernet FastEthernet IEEE 802.3
GigabitEthernet GigabitEthernet IEEE 802.3z
Loopback Loopback interface
Serial Serial
R1(config-if)#tunnel source s0/0/0
R1(config-if)#
R1(config-if)#
R1(config-if)#tunne

12. GRE TUNNEL CONFIGURATION
R1(config-if)#tunnel ?
destination destination of tunnel
mode tunnel encapsulation method
source source of tunnel packets
R1(config-if)#tunnel des
R1(config-if)#tunnel destination ?
A.B.C.D ip address
R1(config-if)#tunnel destination 30.30.30.2
R1(config-if)#
%LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel0, changed state to up

13. GRE TUNNEL CONFIGURATION
R1(config-if)#tun
R1(config-if)#tunnel ?
destination destination of tunnel
mode tunnel encapsulation method
source source of tunnel packets
R1(config-if)#tunnel mo
R1(config-if)#tunnel mode ?
gre generic route encapsulation protocol
ipv6ip IPv6 over IP encapsulation
R1(config-if)#tunnel mode gr
R1(config-if)#tunnel mode gre ?
ip over IP
R1(config-if)#tunnel mode gre ip ?
<cr>
R1(config-if)#tunnel mode gre ip

14. GRE TUNNEL CONFIGURATION
Here is show run so far:
interface Tunnel0
ip address 10.10.10.1 255.255.255.252
tunnel mode gre ip
tunnel source Serial0/0/0
tunnel destination 30.30.30.2
Lets check R1# show ip int brief
R1#show ip int brief

15. GRE TUNNEL CONFIGURATION
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.1.100 YES manual up up
FastEthernet0/1 unassigned YES unset administratively down down
Serial0/0/0 20.20.20.1 YES manual up up
Tunnel0 10.10.10.1 YES manual up up
!

16. GRE TUNNEL CONFIGURATION
Step 2) Now I will do same concept on R2 (make sure use your own interface as
source and use public IP address of R1=20.20.20.1)
interface Tunnel0
ip address 10.10.10.2 255.255.255.252
mtu 1476
tunnel source Serial0/0/0
tunnel destination 20.20.20.1

17. GRE TUNNEL CONFIGURATION
R2#
R2#config t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#int tunnel 0
R2(config-if)#
%LINK-5-CHANGED: Interface Tunnel0, changed state to up
R2(config-if)#
R2(config-if)#ip add
R2(config-if)#ip address 10.10.10.2 255.255.255.252

18. GRE TUNNEL CONFIGURATION
R2(config-if)#
R2(config-if)#tun
R2(config-if)#tunnel ?
destination destination of tunnel
mode tunnel encapsulation method
source source of tunnel packets
R2(config-if)#tunnel sou
R2(config-if)#tunnel source s0/0/0
R2(config-if)#tunnel des
R2(config-if)#tunnel destination 20.20.20.1
R2(config-if)#
%LINEPROTO-5-UPDOWN: Line protocol on Interface
Tunnel0, changed state to up

19. GRE TUNNEL CONFIGURATION
R2(config-if)#trun
R2(config-if)#tun
R2(config-if)#tunnel ?
destination destination of tunnel
mode tunnel encapsulation method
source source of tunnel packets
R2(config-if)#tunnel mo
R2(config-if)#tunnel mode ?
gre generic route encapsulation protocol

20. GRE TUNNEL CONFIGURATION
ipv6ip IPv6 over IP encapsulation
R2(config-if)#tunnel mode gr
R2(config-if)#tunnel mode gre ?
ip over IP
R2(config-if)#tunnel mode gre ip ?
<cr>
R2(config-if)#tunnel mode gre ip
R2(config-if)#

21. GRE TUNNEL CONFIGURATION
Here is show run so far on R2#:
interface Tunnel0
ip address 10.10.10.2 255.255.255.252
mtu 1476
tunnel source Serial0/0/0
tunnel destination 20.20.20.1
Now Lets look at R2#show ip int brief and See if I can ping from R1 to R2 Tunnel IP
address=10.10.10.2
R2#show ip int brief

22. GRE TUNNEL CONFIGURATION
R2#
R2#ping 10.10.10.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.10.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 3/11/18 ms
Interface IP-Address OK? Method Status Protocol
FastEthernet0
/0
192.168.1.100 YES manual up up
FastEthernet0
/1
unassigned YES unset administratively
down
down
Serial0/0/0 30.30.30.2 YES manual up up
Tunnel0 10.10.10.2 YES manual up up

23. GRE TUNNEL CONFIGURATION
R2#ping 10.10.10.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.10.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/17/24 ms
As we see we have connectivity from Tunnel of R1 to Tunnel of R2
But let’s see if I can ping from PC1 to PC2?

24. GRE TUNNEL CONFIGURATION
PC>ping 192.168.2.1
Pinging 192.168.2.1 with 32 bytes of data:
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Ping statistics for 192.168.2.1:
Packets: Sent = 4, Received = 0, Lost = 4 (100%

25. GRE TUNNEL CONFIGURATION
Not yet? Why? Since R1 routing table does not know how to get to network
192.168.2.0 ,
Let’s look at R1#show ip route
R1#show ip route
Codes: C – connected, S – static, I – IGRP, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2, E – EGP
i – IS-IS, L1 – IS-IS level-1, L2 – IS-IS level-2, ia – IS-IS inter area
* – candidate default, U – per-user static route, o – ODR
P – periodic downloaded static route
Gateway of last resort is 20.20.20.2 to network 0.0.0.0

26. GRE TUNNEL CONFIGURATION
10.0.0.0/30 is subnetted, 1 subnets
C 10.10.10.0 is directly connected, Tunnel0
20.0.0.0/30 is subnetted, 1 subnets
C 20.20.20.0 is directly connected, Serial0/0/0
C 192.168.1.0/24 is directly connected,
FastEthernet0/0
S* 0.0.0.0/0 [1/0] via 20.20.20.2

27. GRE TUNNEL CONFIGURATION
Step 3) Make sure create a Static Route on Both R1 and R2 , to be able to see each other
network ; that is R1 need to see network 192.168.2.0 and R2 need to see network 192.168.1.0
Hint: when you put the next hop, make sure use private ip address of each other =10.10.10.1
and 10.10.10.2
Let’s do R1 static route:
R1#config t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#ip route ?
A.B.C.D Destination prefix
R1(config)#ip route 192.168.2.0 ?
A.B.C.D Destination prefix mask

28. GRE TUNNEL CONFIGURATION
R1(config)#ip route 192.168.2.0 255.255.255.0 ?
A.B.C.D Forwarding router’s address
Ethernet IEEE 802.3
FastEthernet FastEthernet IEEE 802.3
GigabitEthernet GigabitEthernet IEEE 802.3z
Loopback Loopback interface
Null Null interface
Serial Serial
Vlan Catalyst Vlans
R1(config)#ip route 192.168.2.0 255.255.255.0
10.10.10.2

29. GRE TUNNEL CONFIGURATION
Here is show run on R1:
R1#show run
ip route 192.168.2.0 255.255.255.0 10.10.10.2
(destination Net) (Next hop)
Here is show ip route on R1#:
R1#show ip route
Codes: C – connected, S – static, I – IGRP, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2, E – EGP
i – IS-IS, L1 – IS-IS level-1, L2 – IS-IS level-2, ia – IS-IS inter area
* – candidate default, U – per-user static route, o – ODR
P – periodic downloaded static route
Gateway of last resort is 20.20.20.2 to network 0.0.0.0

30. GRE TUNNEL CONFIGURATION
10.0.0.0/30 is subnetted, 1 subnets
C 10.10.10.0 is directly connected, Tunnel0
20.0.0.0/30 is subnetted, 1 subnets
C 20.20.20.0 is directly connected, Serial0/0/0
C 192.168.1.0/24 is directly connected, FastEthernet0/0
S 192.168.2.0/24 [1/0] via 10.10.10.2
S* 0.0.0.0/0 [1/0] via 20.20.20.2
R1#

31. GRE TUNNEL CONFIGURATION
Now I will do same static route on R2
R2#config t
Enter configuration commands, one per line. End
with CNTL/Z.
R2(config)#ip route ?
A.B.C.D Destination prefix
R2(config)#ip route 192.168.1.0 ?
A.B.C.D Destination prefix mask
R2(config)#ip route 192.168.1.0 255.255.255.0 ?
A.B.C.D Forwarding router’s address
Ethernet IEEE 802.3

32. GRE TUNNEL CONFIGURATION
FastEthernet FastEthernet IEEE 802.3
GigabitEthernet GigabitEthernet IEEE 802.3z
Loopback Loopback interface
Null Null interface
Serial Serial
Vlan Catalyst Vlans
R2(config)#ip route 192.168.1.0 255.255.255.0 10.10.10.1
R2(config)#

33. GRE TUNNEL CONFIGURATION
Here is my show run on R2#:
ip route 192.168.1.0 255.255.255.0 10.10.10.1
(dest net ) (Next Hop)
Now I should be able to ping from PC1 to PC2 ,
PC>ping 192.168.2.1

34. GRE TUNNEL CONFIGURATION
Pinging 192.168.2.1 with 32 bytes of data:
Reply from 192.168.2.1: bytes=32 time=5ms TTL=126
Reply from 192.168.2.1: bytes=32 time=2ms TTL=126
Reply from 192.168.2.1: bytes=32 time=2ms TTL=126
Reply from 192.168.2.1: bytes=32 time=2ms TTL=126

35. GRE TUNNEL CONFIGURATION
Ping statistics for 192.168.2.1:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 2ms, Maximum = 5ms, Average = 2ms

36. GRE TUNNEL CONFIGURATION
So here is summary of each router:
R1#show run
interface Tunnel0
ip address 10.10.10.1 255.255.255.252
mtu 1476
tunnel source Serial0/0/0 (use your own
tunnel destination 30.30.30.2 ( use Public IP address
R2)
!
!

37. GRE TUNNEL CONFIGURATION
interface FastEthernet0/0
ip address 192.168.1.100 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet0/1
no ip address
duplex auto
speed auto
shutdown
!

38. GRE TUNNEL CONFIGURATION
interface Serial0/0/0
ip address 20.20.20.1 255.255.255.252
clock rate 2000000
!
interface Vlan1
no ip address
shutdown
!
ip classless
ip route 0.0.0.0 0.0.0.0 20.20.20.2
ip route 192.168.2.0 255.255.255.0 10.10.10.2 ( static route to other side
network ; and use private IP
address of Tunnel of R2)

39. GRE TUNNEL CONFIGURATION
Now here is Show run on R2#:
R2#
interface Tunnel0
ip address 10.10.10.2 255.255.255.252
mtu 1476
tunnel source Serial0/0/0 (use your own interface )
tunnel destination 20.20.20.1 ( use the public IP address of R1)
!
!

40. GRE TUNNEL CONFIGURATION
interface FastEthernet0/0
ip address 192.168.2.100 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet0/1
no ip address
duplex auto
speed auto
shutdown
!

41. GRE TUNNEL CONFIGURATION
interface Serial0/0/0
ip address 30.30.30.2 255.255.255.252
clock rate 2000000
!
interface Vlan1
no ip address
shutdown
!
ip classless
ip route 0.0.0.0 0.0.0.0 30.30.30.1
ip route 192.168.1.0 255.255.255.0 10.10.10.1 (static route to other
network with remote Tunnel IP Address)

42. ASM EDUCATIONAL CENTER INC. (ASM)
WHERE TRAINING, TECHNOLOGY & SERVICE CONVERGE
To watch our Cisco CCNA Video Trainings Please Check out the link below:
www.asmed.com/c1
Phone: (301) 984-7400
Rockville,MD