Configuring Site to Site IPSec VPN Tunnel Between Cisco Routers …!!!

Site to Site IPSec VPN Tunnel :

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 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:

1. Phase 1 : creates the first tunnel, which protects later ISAKMP negotiation messages.
2. 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) :

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.

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

#.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 calledcrypto 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

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 : You can assign only one crypto map to an interface .

#. 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