Configuring Site to Site IPSec VPN Tunnel Between Cisco Routers

 

Configuring Site to Site IPSec VPN Tunnel Between Cisco Routers

 

 

 

Site-to-Site IPSec VPN Tunnels are used to allow the secure transmission of data, voice and video between two sites (e.g offices or branches). The VPN tunnel is created over the Internet public network and encrypted using a number of advanced encryption algorithms to provide confidentiality of the data transmitted between the two sites.

 

This article will show how to setup and configure two Cisco routers to create a permanent secure site-to-site VPN tunnel over the Internet, using the IP Security (IPSec) protocol.

 

ISAKMP (Internet Security Association and Key Management Protocol) and IPSec are essential to building and encrypting the VPN tunnel. ISAKMP, also called IKE (Internet Key Exchange), is the negotiation protocol that allows two hosts to agree on how to build an IPsec security association. ISAKMP negotiation consists of two phases: Phase 1 and Phase 2.

 

Phase 1 creates the first tunnel, which protects later ISAKMP negotiation messages. Phase 2 creates the tunnel that protects data. IPSec then comes into play to encrypt the data using encryption algorithms and provides authentication, encryption and anti-replay services.

IPSec VPN Requirements

 

To help make this an easy-to-follow exercise, we have split it into two steps that are required to get the Site-to-Site IPSec VPN Tunnel to work.

 

These steps are:

 

(1) Configure ISAKMP (ISAKMP Phase 1)

 

(2) Configure IPSec (ISAKMP Phase 2, ACLs, Crypto MAP)

 

Our example setup is between two branches of a small company, these are Site 1 and Site 2. Both the branch routers connect to the Internet and have a static IP Address assigned by their ISP as shown on the diagram:

 

 

 

Site 1 is configured with an internal network of 10.10.10.0/24, while Site 2 is configured with network 20.20.20.0/24. The goal is to securely connect both LAN networks and allow full communication between them, without any restrictions.

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.

 

To begin, we’ll start working on the Site 1 router (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 listed 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 set is the default value.

 

We should note that ISAKMP Phase 1 policy is defined globally. This means that if we have five different remote sites and configured five different ISAKMP Phase 1 policies (one for each remote router), when our router tries to negotiate a VPN tunnel with each site it will send all five policies and use the first match that is accepted by both ends.

 

Next we are going to define a pre shared key for authentication with our peer (R2 router) by using the following command:

 

• R1(config)# crypto isakmp key firewallcx address 1.1.1.2

 

The peer’s pre shared key is set to firewallcx and its public IP Address is 1.1.1.2. Every time R1 tries to establish a VPN tunnel with R2 (1.1.1.2), this pre shared key will be used.

Configure IPSec

 

To configure IPSec we need to setup the following in order:

 

- Create extended ACL

 

- Create IPSec Transform

 

- Create Crypto Map

 

- Apply crypto map to the public interface

 

Let us examine each of the above steps.

Creating Extended ACL

 

Next step is to create an access-list and define the traffic we would like the router to pass through the VPN tunnel. In this example, it would be traffic from one network to the other, 10.10.10.0/24 to 20.20.20.0/24. Access-lists that define VPN traffic are sometimes called crypto access-list or interesting traffic access-list.

 

R1(config)# ip access-list extended VPN-TRAFFIC

R1(config-ext-nacl)# permit ip 10.10.10.0 0.0.0.255 20.20.20.0 0.0.0.255

Create IPSec Transform (ISAKMP Phase 2 policy)

 

Next step is to create the transform set used to protect our data. We’ve named this TS:

 

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

 

The above command defines the following:

 

- ESP-3DES - Encryption method

 

- MD5 - Hashing algorithm

Create Crypto Map

 

The Crypto map is the last step of our setup and connects the previously defined ISAKMP and IPSec configuration together:

 

• R1(config)# crypto map CMAP 10 ipsec-isakmp
•  
• R1(config-crypto-map)# set peer 1.1.1.2
•  
• R1(config-crypto-map)# set transform-set TS
•  
• R1(config-crypto-map)# match address VPN-TRAFFIC

 

We’ve named our crypto map CMAP. The ipsec-isakmp tag tells the router that this crypto map is an IPsec crypto map. Although there is only one peer declared in this crypto map (1.1.1.2), it is possible to have multiple peers within a given crypto map.

 

 

Apply Crypto Map to the Public Interface

 

The final step is to apply the crypto map to the outgoing interface of the router. Here, the outgoing interface is FastEthernet 0/1.

 

• R1(config)# interface FastEthernet0/1
• R1(config- if)# crypto map CMAP

 

Note that you can assign only one crypto map to an interface.

 

As soon as we apply crypto map on the interface, we receive a message from the router that confirms isakmp is on: “ISAKMP is ON”.

 

At this point, we have completed the IPSec VPN configuration on the Site 1 router.

 

We now move to the Site 2 router to complete the VPN configuration. The settings for Router 2 are identical, with the only difference being the peer IP Addresses and access lists:

 

• 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
•  
• R2(config)# crypto isakmp key firewallcx address 1.1.1.1
•  
• R2(config)# ip access-list extended VPN-TRAFFIC
•  
• R2(config-ext-nacl)# permit ip 20.20.20.0 0.0.0.255 10.10.10.0 0.0.0.255
•  
• R2(config)# crypto ipsec transform-set TS esp-3des esp-md5-hmac
•  
• R2(config)# crypto map CMAP 10 ipsec-isakmp
•  
• R2(config-crypto-map)# set peer 1.1.1.1
•  
• R2(config-crypto-map)# set transform-set TS
•  
• R2(config-crypto-map)# match address VPN-TRAFFIC
•  
• R2(config)# interface FastEthernet0/1
•  
• R2(config- if)# crypto map CMAP
•  

 

Bringing Up and Verifying the VPN Tunnel

 

At this point, we’ve completed our configuration and the VPN Tunnel is ready to be brought up. To initiate the VPN Tunnel, we need to force one packet to traverse the VPN and this can be achieved by pinging from one router to another:

 

R1# ping 20.20.20.1 source fastethernet0/0

 

Type escape sequence to abort.

 

Sending 5, 100-byte ICMP Echos to 20.20.20.1, timeout is 2 seconds:

 

Packet sent with a source address of 10.10.10.1

 

.!!!!

 

Success rate is 80 percent (4/5), round-trip min/avg/max = 44/47/48 ms

 

 

 

 

 

The first ping received a timeout, but the rest received a reply, as expected. The time required to bring up the VPN Tunnel is sometimes slightly more than 2 seconds, causing the first ping to timeout.

 

To verify the VPN Tunnel, use the show crypto session command:

 

R1# show crypto session

 

Crypto session current status

 

Interface: FastEthernet0/1

 

Session status: UP-ACTIVE

 

Peer: 1.1.1.2 port 500

 

IKE SA: local 1.1.1.1/500 remote 1.1.1.2/500 Active

 

IPSEC FLOW: permit ip 10.10.10.0/255.255.255.0 20.20.20.0/255.255.255.0

 

Active SAs: 2, origin: crypto map

 

 

Network Address Translation (NAT) and IPSec VPN Tunnels

 

Network Address Translation (NAT) is probably configured to provide Internet access to internal hosts. When configuring a Site-to-Site VPN tunnel, it is imperative to instruct the router not to perform NAT (deny NAT) on packets destined to the remote VPN network.

 

This is easily done by inserting a deny statement at the beginning of the NAT access lists as shown below:

 

For Site 1’s router:

 

• R1(config)# ip nat inside source list 100 interface fastethernet0/1 overload
•  
• R1(config)# access-list 100 remark -=[Define NAT Service]=-
•  
• R1(config)# access-list 100 deny ip 10.10.10.0 0.0.0.255 20.20.20.0 0.0.0.255
•  
• R1(config)# access-list 100 permit ip 10.10.10.0 0.0.0.255 any
•  
• R1(config)# access-list 100 remark

 

 

 

And Site 2’s router:

 

• R2(config)# ip nat inside source list 100 interface fastethernet0/1 overload
•  
• R2(config)# access-list 100 remark -=[Define NAT Service]=-
•  
• R2(config)# access-list 100 deny ip 20.20.20.0 0.0.0.255 10.10.10.0 0.0.0.255
•  
• R2(config)# access-list 100 permit ip 20.20.20.0 0.0.0.255 any
•  
• R2(config)# access-list 100 remark