OVERLAPPING SUBNET IPSEC VPN

This blog provides a networking example that simulates two different companies with the same IP addressing scheme. Two routers are connected with a VPN tunnel, and the networks behind each router are the same. For one site to access hosts at the other site, Network Address Translation (NAT) with route map is used on the routers to change both the source and the destination addresses to different subnets.

TEST RTR CONFIGURATION

IKE PHASE I

crypto isakmp policy 10

encr 3des

hash md5

authentication pre-share

group 5

crypto isakmp key cisco123 address 202.45.144.123

IKE PHASE II

crypto ipsec transform-set set-test esp-des esp-sha-hmac

crypto map mymap 10 ipsec-isakmp

set peer 202.45.144.123

set transform-set set-test

match address IPSEC-VPN

reverse-route static

NAT CONFIG

ip nat inside source static network 10.112.112.0 172.16.2.0 /24

Note : As our subnet is same in both the end NAT must be performed otherwise not required.

ACCESS-LIST

ip access-list extended IPSEC-VPN

permit ip 172.16.2.0 0.0.0.255 172.16.1.0 0.0.0.255

MOF RTR CONFIGURATION

IKE PHASE I

crypto isakmp policy 81

encr 3des

hash md5

authentication pre-share

group 5

crypto isakmp key cisco123 address 202.166.195.212

IKE PHASE II

crypto ipsec transform-set set-mof esp-des esp-sha-hmac

crypto map mymap 81 ipsec-isakmp

set peer 202.166.195.212

set transform-set set-mof

match address VPN-MAX

reverse-route static

ACCESS-LIST

Extended IP access list VPN-MAX

10 permit ip 172.16.1.0 0.0.0.255 host 172.16.2.10

NAT

ip nat inside source static 10.112.112.16 172.16.1.16

NOTE : NAT IS BEING USED AS LOCAL SUBNET ON BOTH THE END IS SAME.

One to One Nat is being currently performed in both the sides for the address to be translated which could bring out issues if we are pairing up with other remote sites as well as in the case of using internet for that particular machine. So, in order to overcome the issue we must be using route map and define a new set of ACL to be called up in the route map

ACCESS-LIST FOR ROUTE MAP

Extended IP access list VPN-SCB-TEST

10 permit ip 10.112.112.0 0.0.0.255 host 172.16.2.10

172.16.2.10 is the remote NAT'd IP which will be the source IP if the traffic is initiated from the TEST RTR side

ROUTE MAP

route-map RM-STATIC-NAT permit 10

match ip address VPN-SCB-TEST

NAT with Route MAP

ip nat inside source static 10.112.112.16 172.16.1.16 route-map RM-STATIC-NAT

I have only demonstrated NAT with route map example in one of the sites but route map needs to be placed on both the sides.

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