site to site VPN overlapping subnets on Cisco ASA 5540

gflorescu · ‎06-23-2009

I am looking for some advice on how to properly setup a site to site VPN when there's overlapping subnets.

i.e. if you have 172.16.x.x on both sides of the tunnel.

I have set this up before and here's my config but I'm not sure if this is the best way to do it.

access-list outboundpolicy_NAT extended permit ip 172.31.21.0 255.255.255.128 19

2.168.0.0 255.255.248.0

static (outside,inside) 192.168.0.0 172.16.0.0 netmask 255.255.248.0

static (inside,outside) 192.168.0.0 access-list outboundpolicy_NAT

On the VPN tunnel, I configured this ACL:

access-list tac-VPN-domain extended permit ip 192.168.0.0 255.255.248.0 172.16.0.0 255.255.248.0

Patrick0711 · ‎06-23-2009

If both sides are using identical encryption domains, you'll need to:

a.) Configure Policy NAT on both ends of the tunnel

b.) Use a Public <-> Private or Public <-> Public VPN connection

c.) Change the internal subnet of one firewall.

The key thing to remember about option A it that you have to policy NAT on both sides since the subnets are identical:

Example of config on ASA:

access-list Policy_NAT extended permit ip

static (inside,outside) access-list Policy_NAT

access-list crypto_ACL extended permit ip

Also remember that the policy NAT static statement will need to be located above any other static NAT statements.