configuring point-to-point gre vpn tunnels - unprotected gre & protected gre over ipsec tunnels

8/10/2019 Configuring Point-To-Point GRE VPN Tunnels - Unprotected GRE & Protected GRE Over IPSec Tunnels

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9/23/13 Configuring Point- to-Point GRE VPN Tunnels - Unprotected GRE & Protected GRE over IPSec Tunnels

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Configuring Point-to-Point GRE VPN Tunnels - Unprotected GRE &Protected GRE over IPSec TunnelsWritten by Administrator

Friday, 04 May 2012 21:10

Generic Routing Encapsulation (GRE) is a tunneling protocol developed by Cisco that allows the encapsulation of a widevariety of network layer protocols inside point-to-point links.

A GRE tunnel is used when packets need to be sent from one network to another over the Internet or an insecure network.

With GRE, a virtual tunnel is created between the two endpoints (Cisco routers) and packets are sent through the GRE

tunnel.

It is important to note that packets travelling inside a GRE tunnel are not encrypted as GRE does not encrypt the tunnel but

encapsulates it with a GRE header. If data protection is required, IPSec must be configured to provide data confidentiality

this is when a GRE tunnel is transformed into a secure VPN GRE tunnel.

The diagram below shows the encapsulation procedure of a simple - unprotected GRE packet as it traversers the router

and enters the tunnel interface:

Whilemany might think a GRE IPSec tunnel between two routers is sim ilar to a si te to site IPSec VPN (crypto), it is not. A

major difference is that GRE tunnels allow multicast packets to traverse the tunnel whereas IPSec VPN does not support

multicast packets. In large networks where routing protocols such as OSPF, EIGRP are necess ary, GRE tunnels are your

best bet. For this reason, plus the fact that GRE tunnels are much eas ier to configure, engineers preferto use GRE rather

than IPSec VPN.

This article will explain how to create sim ple (unprotected) and secure (IPSec encrypted) GRE tunnels between endpoints.

We explain all the necessary steps to create and verify the GRE tunnel (unprotected and protected) and configure routing

between the two networks.

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Creating a Cisco GRE Tunnel

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.

First step is to create our tunnel interface on R1:

R1(config)# interface Tunnel0

R1(config-if)# ip address 172.16.0.1 255.255.255.0

R1(config-if)# ip mtu 1400

R1(config-if)# ip tcp adjust-mss 1360

R1(config-if)# tunnel source 1.1.1.10

R1(config-if)#tunnel destination 2.2.2.10

All Tunnel interfaces of participating routers must always be configured with an IP address that is not used anywhere elsein the network. Each Tunnel interface is as signed an IP address within the same network as the other Tunnel interfaces.

In our example, both Tunnel interfaces are part of the 172.16.0.0/24network.

Since GRE is an encapsulating protocol, we adjust the maximum transfer unit(mtu) to 1400 bytes and maximum segment

size(mss) to 1360 bytes. Because most transport MTUs are 1500 bytes and we have an added overhead because of

GRE, we must reduce the MTU to account for the extra overhead. A setting of 1400 is a common practice and will ensure

unnecessary packet fragmentation is kept to a minimum.

Closing, we define the Tunnel source, which is R1s public IP address, and destination R2s public IP address

As soon as we complete R1s configuration, the router will confirm the creation of the tunnel and inform about its s tatus:

R1#

*May 4 21:30:22.971: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel0, changed s tate to up

Since the Tunnel 0 interface is a logical interface it will remain up even if there is no GRE tunnel configured or connected at

the other end.

Next, we must create the Tunnel 0interface on R2:

R2(config)# interface Tunnel0

R2(config-if)# ip address 172.16.0.2 255.255.255.0

R2(config-if)# ip mtu 1400

R2(config-if)# ip tcp adjust-mss 1360

R2(config-if)#tunnel source 2.2.2.10

R2(config-if)# tunnel destination 1.1.1.10
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R2s Tunnel interface is configured with the appropriate tunnel source and destination IP address. As with R1, R2 router

will inform us that the Tunnel0interface is up:

R2#

*May 4 21:32:54.927: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel0, changed s tate to up

Routing Networks Through the GRE Tunnel

At this point, both tunnel endpoints are ready and can s ee each other. An icmp echo from one end will confirm this :

R1# ping 172.16.0.2

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.16.0.2, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

R1#

Again, this result means that the two tunnel endpoints can see each other. Workstations on either network wil l sti ll not be

able to reach the other side unles s a s tatic route is placed on each endpoint:

R1(config)# ip route 192.168.2.0 255.255.255.0 172.16.0.2

On R1 we add a static route to the remote network 192.168.2.0/24 via 172.16.0.2 which is the other end of our GRE Tunnel.

When R1 receives a packet for 192.168.2.0 network, it now knows the next hop is 172.16.0.2 and therefore will send it

through the tunnel.

The same configuration must be repeated for R2:

R2(config)# ip route 192.168.1.0 255.255.255.0 172.16.0.1

Now both networks are able to freely communicate with each over the GRE Tunnel.

Securing the GRE Tunnel with IPSec

As mentioned earlier, GRE is an encaps ulation protocol and does not perform any encryption. Creating a point-to-point

GRE tunnel without any encryption is extremely risky as sens itive data can easi ly be extracted from the tunnel and viewed

by others.

For this purpose, we use IPSec to add an encryption layer and secure the GRE tunnel. This provides us with the necessary

military-grade encryption and peace of mind. Our example below covers GRE IPSec Tunnel mode.

GRE IPSec modes are covered extensively in our GRE and IPSec GRE Over IPSec - Selecting and Configuring Gre IPSec

Tunnel or Transport Mode.

Configuring IPSec Encryption for GRE Tunnel (GRE over IPSec)

IPSec encryption involves two steps for each router. These steps are:

(1) Configure ISAKMP (ISAKMP Phase 1)

(2) Configure IPSec (ISAKMP Phas e 2)

Configure ISAKMP (IKE) - (ISAKMP Phase 1)

IKE exists only to establish SAs (Security Association) for IPsec. Before it can do this, IKE must negotiate an SA (an

ISAKMP SA) relationship with the peer.
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To begin, well s tart working on R1.

First step is to configure an ISAKMP Phase 1 policy:

R1(config)# crypto isakmp policy 1

R1(config-isakmp)# encr 3des

R1(config-isakmp)# hash md5

R1(config-isakmp)# authentication pre-share

R1(config-isakmp)# group 2

R1(config-isakmp)# lifetime 86400

The above commands define the following (in lis ted order):

3DES- The encryption method to be used for Phase 1.

MD5- The hashing algorithm

Pre-share- Use Pre-shared key as the authentication method

Group 2- Diffie-Hellman group to be used

86400 Session key lifetime. Expressed in either kilobytes (after x-amount of traffic, change the key) or seconds. Value s et

is the default value.

Next we are going to define a pre s hared key for authentication with R1's peer, 2.2.2.10:

R1(config)# crypto isakmp key firewallcx address 2.2.2.10

The peers pre shared key is s et to firewallcx. This key will be used for allISAKMP negotiations with peer 2.2.2.10 (R2).

Create IPSec Transform (ISAKMP Phase 2 policy)

Now we need to create the transform set us ed to protect our data. Weve named this TS:

R1(config)# crypto ipsec transform-set TS esp-3des esp-md5-hmac

R1(cfg-crypto-trans)# mode transport

The above commands defines the following:

- ESP-3DES- Encryption method

- MD5 - Hashing algorithm

- Set IPSec to transportmode

Finally, we create an IPSec profile to connect the previously defined ISAKMP and IPSec configuration together. Weve

named our IPSec profile protect-gre:

R1(config)# crypto ipsec profile protect-gre

R1(ipsec-profile)# set security-association lifetime seconds 86400

R1(ipsec-profile)# set transform-set TS

We are ready to apply the IPSec encryption to the Tunnel interface:

R1(config)# interface Tunnel 0

R1(config-if)# tunnel protection ipsec profile protect-gre

Now it's time to apply the same configuration on R2:

R2(config)# crypto isakmp policy 1

R2(config-isakmp)# encr 3des

R2(config-isakmp)# hash md5

R2(config-isakmp)# authentication pre-share

R2(config-isakmp)# group 2

R2(config-isakmp)# lifetime 86400


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R2(config)# crypto isakmp key firewallcx address 1.1.1.10

R2(config)# crypto ipsec transform-set TS esp-3des esp-md5-hmac

R2(cfg-crypto-trans)# mode transport

R2(config)# crypto ipsec profile protect-gre

R2(ipsec-profile)# set security-association lifetime seconds 86400

R2(ipsec-profile)# set transform-set TS

R2(config)# interface Tunnel 0

R2(config-if)# tunnel protection ipsec profile protect-gre

Verifying the GRE over IPSec Tunnel

Finally, our tunnel has been encrypted with IPSec, providing us with the much needed security layer. To test and verify this,

all that is required is to ping the other end and force the VPN IPSec tunnel to come up and start encrypting/decrypting our

data:

R1#ping 192.168.2.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms

Using the show crypto sessioncommand, we can quickly verify the encryption is in place and doing i ts work:

R1#show crypto session

Crypto session current status

Interface: Tunnel0

Session status: UP-ACTIVE

Peer: 2.2.2.10 port 500

IKE SA: local 1.1.1.10/500 remote 2.2.2.10/500 Active

IPSEC FLOW: permit 47 host 1.1.1.10 host 2.2.2.10

Active SAs: 2, origin: crypto map

Add a comment28 comments

Alan Cameron Managing Director at Cameron IT

Solutions

Great Guideto GRE Tunnels, although I am havingtrouble securing the gre tunnel (setup a labenvironment) once I setup ipsec as above I can nolonger ping the overside of the tunnel. tried putt ingin access-lists to allow the gre tunnel etc but to noavail.any ideas would be great.

Reply Like 22 June at 17:171

Chris Partsenidis Top commenter Founder, Editor-in-Chief at Firewall.cx

Thanks for the feedback Alan. Normallyyou shouldn't have these kind ofproblems. GRE tunnels are prettystraight forward and usually work 'first-
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9/23/13 Configuring Point-to-Point GRE VPN Tunnels - Unprotected GRE & Protected GRE over IPSec Tunnels


go . n e ryp o sec unne s, eydo not require 'no-nat' access lists. I'dadvise to check your configurationagain and pay attention to yourrouting, perhaps you missedsomethingthere; Good luck and feel free to 'Like'us and share the site with others!

Reply Like 22 June at 22:501

Dan Anderson Kansas City, Missouri

Good grief this page saved me a lot of time -thanks.

Reply Like 22 May at 10:332

Chris Partsenidis Top commenter

Founder, Editor-in-Chief at Firewall.cx

Glad to hear it helped you Dan - Feelfree to share it! Thanks,

Reply Like 22 Mayat 11:48

Andy Mba Stockholm, SwedenWell explaned, easy to understand.

Reply Like 18 December 2012 at 04:122

Umer Muhammad

Wow, Great.. .this really helped me in field work.

Reply Like 4 November 2012 at 01:281

Mihai Parasca Universitatea Tehnic Cluj-Napoca

Amazingly clear!Thank you!

Reply Like 27 October 2012 at 20:181

Chris Partsenidis Top commenter

Founder, Editor-in-Chief at Firewall.cx

Thanks Mihai for your feedback! Pleasedon't forget to 'like' our articles and site:)

Reply Like 28 October 2012 at

03:24

1

Aamir Sadiq Aalim Muhammed Salegh College of

Engineering

helpful indeed, many thanks.

Reply Like 19 August 2012 at 06:223

Mudassar Jaleel Works at Verizon

Wireless

kya bhai,getting in tunnel :)


Ahmed Said Top commenter

You saved my ass.

Reply Like 6 September 2012 at 01:242

Karthick Sivarajan Works at Tata Consultancy
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7/7



Last Updated on Sunday, 13 May 2012 21:42

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F acebook social plugin

Services

Very usefull and well organised description!


Rahul Singh

Excellent

Reply Like 8 May 2012 at 04:033

Thiasma Thithi Institut suprieur de tecnologies

Siantou

Smart configuration... . (^_^)


Foster Lavrov Devant ma cours

Man. Ne nous blase plus avec les wayfort. Mboutman

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configuring point-to-point gre vpn tunnels - unprotected gre & protected gre over ipsec tunnels

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