Site to Site VPN with 2 ASA 5510's

Question

Hello guys,
Im hoping yall can help me with the following objective. I have been tasked to make a site to site VPN between two networks. We are both using an ASA 5510.
This is the scenario:
SiteA has an wan adress of (example) 20.20.20.20  - The firewall is connected to a DMZ range : 192.168.0.0 255.255.255.0. In this range there is another firewall which grants/blocks acces to the internal range. 10.20.0.0 255.255.0.0
SiteB has an wan adress of (example) 21.21.21.21 - The internal range is 10.0.0.0 255.0.0.0 No DMZ.
How can i connect these 2 devices since there is an overlap. I am gonna need to use nat right? Can someone give me readable  Access rule/Nat Rule and maybe advice / some other things i need to think of.
Hope to hear from yall. Any advice is highly appriciated.
Thanks in advance

Answer 1

Hi,
Well regarding the remote site I suppose if they are using hosts from ranges 10.0.1.0/24 and 10.0.2.0/24 they could simply NAT these portions of the network towards the L2L VPN connection. For example NAT them to subnets 192.168.101.0/24 and 192.168.102.0/24
But you also seem to have a large subnet on your side since its 10.20.0.0/16. Because of this I would suggest narrowing it down to the hosts or smaller subnets like above with the remote site because simply NATing the whole subnet 10.20.0.0/16 to some other private range that is NOT from the 10.0.0.0/8 range would probably cause problems in the long run.
Lets presume that on your side the network that needs to access the L2L VPN is 10.20.1.0/24 and we would NAT that to 192.168.201.0/24 then your NAT configuration could look like this
object network DMZ-INTERNAL
  subnet 10.20.1.0 255.255.255.0
object network DMZ-INTERNAL-NAT
 subnet 192.168.201.0 255.255.255.0
object-group network REMOTE-NETWORKS
 network-object 192.168.101.0 255.255.255.0
 network-object 192.168.102.0 255.255.255.0
nat (dmz,outside) 1 source static DMZ-INTERNAL DMZ-INTERNAL-NAT destination static REMOTE-NETWORKS REMOTE-NETWORKS
In the above configuration we first create an "object" for both the actual internal DMZ subnet and the subnet that we will NAT it to. Then we create an "object-group" that will have inside it both of the remote NATed networks (NAT performed at the remote site).
Finally the "nat" command itself will perform NAT between "dmz" and "outside" interface and it will NAT "DMZ-INTERNAL" to "DMZ-INTERNAL-NAT" when the destination is "REMOTE-NETWORKS". The NAT configuration is bidirectional so it naturally handles which ever directin the connection is attempted. The names of the objects are up to the user.
The ACL that defines the local and remote networks for the L2L VPN should use the NAT subnets of each site.
If you want to restrict the traffic from the remote site then this can be done in a couple of ways. At its default settings the ASA will allow ALL traffic from the remote site behind the L2L VPN connection.
You can use the command "show run all sysopt" to list some configurations that will tell us how your ASA has been set to handle VPN related traffic. The command we are looking for is "sysopt connection permit-vpn". This is the default setting that allows all traffic from VPN connections. If you were to change this to "no sysopt connection permit-vpn" then you could simply use the interface ACL of the interface that terminates the L2L VPN connection on your side to select what traffic is allowed. You would allow traffic the same way as if you were allowing traffic from Internet to your servers.
The problem with this setup is if you have other existing VPN connections (VPN Client and L2L VPN) because they would also require their traffic to be allowed in your external interfaces ACL if you changed the above mentioned global setting.
The other option is to configure a VPN Filter ACL that you will then attach to a "group-policy". You will then attach that "group-policy" to the "tunnel-group" of the L2L VPN connection.
The actual ACL used for the VPN Filter purpose is a norma ACL but you will always have to configure the remote network as the source in the ACL and this usually causes some confusion.
- Jouni