Oct 20, 2016

How to Configure Site to Site IPSec VPN Tunnel in Cisco IOS Router

IPSec VPN is a security feature that allow you to create secure communication  link (also called VPN Tunnel) between two different networks located at different sites. Cisco IOS routers can be used to setup VPN tunnel between two sites. Traffic like data, voice, video, etc. can be securely transmitted through the VPN tunnel. In this article, I will show you the steps to Configure Site to Site IPSec VPN Tunnel in Cisco IOS Router.






Configure Site to Site IPSec VPN Tunnel in Cisco IOS Router

Diagram below shows our simple scenario. The two sites have static public IP address as shown in the diagram. R1 is configured with 70.54.241.1/24 and R2 is configured with 199.88.212.2/24 IP address. As of now, both routers have very basic setup like, IP addresses, NAT Overload, default route, hostnames, SSH logins, etc.

 
There are two phases in IPSec configuration called Phase 1 and Phase 2. Let’s start the configuration with R1. Before you start configuring the IPSec VPN, make sure both routers can reach each other. I have already verified that both routers can ping each other so let’s start the VPN configuration.

Step 1. Configuring IPSec Phase 1 (ISAKMP Policy)

R1(config)#crypto isakmp policy 5
R1(config-isakmp)#hash sha
R1(config-isakmp)#authentication pre-share
R1(config-isakmp)#group 2
R1(config-isakmp)#lifetime 86400
R1(config-isakmp)#encryption 3des
R1(config-isakmp)#exit
R1(config)#crypto isakmp key cisco@123 address 199.88.212.2

Step 2. Configuring IPSec Phase 2 (Transform Set)

R1(config)#crypto ipsec transform-set MY-SET esp-aes 128 esp-md5-hmac
R1(cfg-crypto-trans)#crypto ipsec security-association lifetime seconds 3600

Step 3. Configuring Extended ACL for interesting traffic.

R1(config)#ip access-list extended VPN-TRAFFIC
R1(config-ext-nacl)#permit ip  192.168.1.0 0.0.0.255 192.168.2.0 0.0.0.255

This ACL defines the interesting traffic that needs to go through the VPN tunnel. Here, traffic originating from 192.168.1.0 network to 192.168.2.0 network will go via VPN tunnel. This ACL will be used in Step 4 in Crypto Map.

Step 4. Configure Crypto Map.

R1(config)#crypto map IPSEC-SITE-TO-SITE-VPN 10 ipsec-isakmp 
% NOTE: This new crypto map will remain disabled until a 
peer and a valid access list have been configured.
R1(config-crypto-map)#match address VPN-TRAFFIC
R1(config-crypto-map)#set peer 199.88.212.2
R1(config-crypto-map)#set transform-set MY-SET


Step 5. Apply Crypto Map to outgoing interface of R1.

R1(config)#int fa0/0
R1(config-if)#crypto map IPSEC-SITE-TO-SITE-VPN
*Mar  1 05:43:51.114: %CRYPTO-6-ISAKMP_ON_OFF: ISAKMP is ON


Step 6. Exclude VPN traffic from NAT Overload.

R1(config)#ip access-list extended 101
R1(config-ext-nacl)#deny ip 192.168.1.0 0.0.0.255 192.168.2.0 0.0.0.255
R1(config-ext-nacl)#permit ip 192.168.1.0 0.0.0.255 any
R1(config-ext-nacl)#exit
R1(config)#ip nat inside source list 101 interface FastEthernet0/0 overload

Above ACL 101 will exclude interesting traffic from NAT.
Now, repeat same steps in R2.

Step 1. Configuring IPSec Phase 1 (ISAKMP Policy)

R2(config)#crypto isakmp policy 5
R2(config-isakmp)#hash sha
R2(config-isakmp)#authentication pre-share
R2(config-isakmp)#group 2
R2(config-isakmp)#lifetime 86400
R2(config-isakmp)#encryption 3des
R2(config-isakmp)#exit
R2(config)#crypto isakmp key cisco@123 address 70.54.241.2


Step 2. Configuring IPSec Phase 2 (Transform Set)

R2(config)#crypto ipsec transform-set MY-SET esp-aes 128 esp-md5-hmac
R2(cfg-crypto-trans)#crypto ipsec security-association lifetime seconds 3600


Step 3. Configuring Extended ACL for interesting traffic.

R2(config)#ip access-list extended VPN-TRAFFIC
R2(config-ext-nacl)#permit ip  192.168.2.0 0.0.0.255 192.168.1.0 0.0.0.255


Step 4. Configure Crypto Map.

R2(config)#crypto map IPSEC-SITE-TO-SITE-VPN 10 ipsec-isakmp
% NOTE: This new crypto map will remain disabled until a peer
        and a valid access list have been configured.
R2(config-crypto-map)#match address VPN-TRAFFIC
R2(config-crypto-map)#set peer 70.54.241.2
R2(config-crypto-map)#set transform-set MY-SET


Step 5. Apply Crypto Map to outgoing interface

R2(config)#int fa0/1
R2(config-if)#crypto map IPSEC-SITE-TO-SITE-VPN
*Mar 1 19:16:14.231: %CRYPTO-6-ISAKMP_ON_OFF: ISAKMP is ON


Step 6. Exclude VPN traffic from NAT Overload.

R1(config)#ip access-list extended 101
R1(config-ext-nacl)#deny ip 192.168.2.0 0.0.0.255 192.168.1.0 0.0.0.255
R1(config-ext-nacl)#permit ip 192.168.2.0 0.0.0.255 any
R1(config-ext-nacl)#exit
R1(config)#ip nat inside source list 101 interface FastEthernet0/1 overload


Verification and testing.
To test the VPN connection let’s ping from R1 to PC2.

R1#ping 192.168.2.1 source 192.168.1.254

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds:
Packet sent with a source address of 192.168.1.254
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 52/54/56 ms


As you can see, the ping from R1 to PC2 is successful. Don’t forget to ping from inside IP address while testing the VPN tunnel from the router. You can also ping from PC1 to PC2.

To verify the IPSec Phase 1 connection, type show crypto isakmp sa as shown below.

R1#show crypto isakmp sa
dst             src             state          conn-id slot status
70.54.241.2     199.88.212.2    QM_IDLE              1    0 ACTIVE

To verify IPSec Phase 2 connection, type show crypto ipsec sa as shown below.

R1#show crypto ipsec sa

interface: FastEthernet0/0
    Crypto map tag: IPSEC-SITE-TO-SITE-VPN, local addr 70.54.241.2

   protected vrf: (none)
   local  ident (addr/mask/prot/port): (192.168.1.0/255.255.255.0/0/0)
   remote ident (addr/mask/prot/port): (192.168.2.0/255.255.255.0/0/0)
   current_peer 199.88.212.2 port 500
     PERMIT, flags={origin_is_acl,}
    #pkts encaps: 9, #pkts encrypt: 9, #pkts digest: 9
    #pkts decaps: 9, #pkts decrypt: 9, #pkts verify: 9
    #pkts compressed: 0, #pkts decompressed: 0
    #pkts not compressed: 0, #pkts compr. failed: 0
    #pkts not decompressed: 0, #pkts decompress failed: 0
    #send errors 16, #recv errors 0

     local crypto endpt.: 70.54.241.2, remote crypto endpt.: 199.88.212.2
     path mtu 1500, ip mtu 1500, ip mtu idb FastEthernet0/0
     current outbound spi: 0xD41CAB1(222415537)

     inbound esp sas:
      spi: 0x9530FB4E(2503015246)
        transform: esp-aes esp-md5-hmac ,

You can also view active IPSec sessions using show crypto session command as shown below.

R1#show crypto session
Crypto session current status

Interface: FastEthernet0/0
Session status: UP-ACTIVE
Peer: 199.88.212.2 port 500
  IKE SA: local 70.54.241.2/500 remote 199.88.212.2/500 Active
  IPSEC FLOW: permit ip 192.168.1.0/255.255.255.0 192.168.2.0/255.255.255.0
        Active SAs: 2, origin: crypto map





We  have done configuring Site to Site IPSec VPN tunnel in Cisco IOS Router.

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