Site to Site VPN enables an encrypted connection between private networks over a public network (i.e. the Internet).

Basic steps to configure a site-to-site VPN with a Cisco ASA begin with defining the ISAKMP Policy. An ISAKMP/IKE policy defines how a connection is to be created, authenticated, and protected. You can have multiple policies on your Cisco ASA. You might need to do this if your ASA needs to connect to multiple devices with different policy configurations.

• Authentication: specifies the method to use for device authentication
• Hash: specifies the HMAC function to use
• Encryption: specifies which algorithm to use
• Group: specifies the DH key group to use

Next, you will need to establish IPsec transform set. Different Firmware versions and different Cisco devices have different options for the following…

• Esp-md5-hmac: ESP with the MD5 (HMAC variant) authentication algorithm
• Esp-aes: ESP with the 128-bit Advanced Encryption Standard (AES) encryption algorithim.
• Esp-des: ESP with the 56-bit Data Encryption Standard (DES) encryption algorithm.
• Esp-3des: ESP with the 168-bit DES encryption algorithm (3DES or Triple DES)
• Ah-md5-hmac: AH with the MD5 (HMAC variant) authentication algorithm
• Ah-sha-hmac: AH with the SHA (HMAC variant) authentication algorithm

3. Configure crypto access list-

Crypto ACL’s are used to identify which traffic is to be encrypted and which traffic is not. After the ACL is defined, the crypto maps use the ACL to identify the type of traffic that IPSec protects.

It’s not recommended to use the permit ip any any command. It causes all outbound traffic to be encrypted, and sends all traffic to the specified peer.

4. Configure crypto map

Used to verify the previously defined parameters

5. Now apply crypto map to the outside interface.

Configuration of ASA-1

You might have to enable ISAKMP on your device

ASA-1(config)#crypto isakmp enable

First defined the IKE polices on ASA-1

ASA-1(config)#crypto isakmp policy 10

The lower the policy number, the higher the priority it will set the ISAKMP policy to, affecting which policies will be used between sites.

General rule of thumb is to give the most secure policy the lowest number (like 1) and the least secure policy the highest number (like 10000)

ASA-1(config-isakmp)#encryption des

(enable encryption des)

ASA-1(config-isakmp)#hash md5

(enable algorithm md5 for hashing)

ASA-1(config-isakmp)#authentication pre-share

(enable Pre-shared method)

ASA-1(config-isakmp)#group 2

(enable group 2)

ASA-1(config-isakmp)#exit

(Exit from crypto isakmp mode)

• The next step is to create a pre-shared key (password) on ASA-1.

ASA-1(config)#crypto isakmp key office address 10.1.1.2

(Here the Key is “office” and 10.1.1.2 is ASA-2 Address)

• Now create an access list to define only interesting traffic.

ASA-1(config)#access-list 100 permit ip host 10.1.1.1 host 10.1.1.2

(100 is access list number and 10.1.1.1 is source address and 10.1.1.2 is destination address.)

• Now create the transform-set for encryption and hashing.

ASA-1(config)#crypto ipsec transform-set ts2 esp-des esp-md5-hmac

(Here encryption type is des and hashing method is md5-hmac)

ASA-1(config)#crypto map testcryp 10 ipsec-isakmp

(crypto map name testcryp)

ASA-1(config)# crypto map testcryp 10 match address 100

(apply the access list)

ASA-1(config)# crypto map testcryp 10 set transform-set ts2

(apply the transform set)

ASA-1(config)# crypto map testcryp 10 set peer 10.1.1.2

(Set remote peer address)

• Now apply the crypto map to the ASA – A interface

ASA-1(config)# crypto map testcryp interface outside

(Apply crypto map on outside interface)

ASA-1(config)# crypto isakmp enable outside

(To enable crypto isakmp on ASA)

Configuration of ASA-2

First defined the IKE polices on ASA-2

ASA-2(config)#crypto isakmp policy 10

(10 is isakmp policy number)

ASA-2(config-isakmp)#encryption des

(enable encryption des)

ASA-2(config-isakmp)#hash md5

(enable algorithm md5 for hashing)

ASA-2(config-isakmp)#authentication pre-share

(enable Pre-shared method)

ASA-2(config-isakmp)#group 2

(enable diffie-Helman group 2)

ASA-2(config-isakmp)#exit

(Exit from crypto isakmp mode)

• The next step is to create a pre-shared key (password) on ASA – B.

ASA-2(config)#crypto isakmp key office address 10.1.1.1

(Here Key is “office” and 10.1.1.1 is ASA – A Address)

• Now create an access list to define only interesting traffic.

ASA-2(config)#access-list 100 permit ip host 10.1.1.2 host 10.1.1.1

(100 is access list number and 10.1.1.2 is source address and 10.1.1.1 is destination address.)

• Now create the transform-set for encryption and hashing.

ASA-2(config)#crypto ipsec transform-set ts2 esp-des esp-md5-hmac

(Here encryption type is des and hashing technique is md5-hmac)

ASA-2(config)#crypto map testcryp 10 ipsec-isakmp

(crypto map name testcryp)

ASA-2(config)# crypto map testcryp 10 match address 100

(apply the access list)

ASA-2(config)# crypto map testcryp 10 set transform-set ts2

(apply the transform set)

ASA-2(config)# crypto map testcryp 10 set peer 10.1.1.1

(Set remote peer address)

• Now apply the crypto map to the ASA – B outside interface

ASA-2(config)# crypto map testcryp interface outside

(Apply crypto map on outside interface)

ASA-2(config)# crypto isakmp enable outside

(To enable crypto isakmp on ASA)

Now to verify the secure tunnel, ping to other remote location.

ASA-2(config)# ping 10.1.1.1

• Some of the material and definitions for this article were referenced from: http://fengnet.com/book/vpnconf/ch16lev1sec2.html
• http://www.cisco.com/en/US/docs/ios/12_2t/12_2t13/feature/guide/ft_aes.html