Siddhartha Jain> writes:

> Hello,
> I am trying to get a LAN-to-LAN IPSec VPN to work.
> Site A is
> Site B is
> On Site A, the inside network accesses the internet by
> being PAT-ted to a pool of four global IP addresses -
> Site B has NAT-ted the hosts to be connected to over
> the VPN with
> Now my question is that how do I configure Site A
> router wrt to NAT.
> Will it work if I leave the PAT on Site A as it is and
> define my interesting traffic as:
> access-list 190 permit ip host
> The PAT on site A is defined as:
> ip nat pool tcsux
> prefix-length 29
> ip nat inside source list 163 pool tcsux overload
> On Site B, the interesting traffic would then be
> between and
> Will this work? Ofcourse, I can punch in the config
> and see if it works but unfortunately Site B isn't
> under my command so I need to suggest the config to
> the Site B admin.
> Thanks,
> Siddhartha Jain (CISSP)
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Hi, with a site to site tunnel, you need to avoid NAT when communicating with
the remote peer and remote networks. Also, you should setup rules to allow
traffic between the internal nets locally and remote....not external nets. So,
you would have an ACL allowing all traffic to go bidirectional from to,; then just make sure you have
static routes setup for the remote networks.

The PAT is fine as long as it does not apply to the remote peer gateway or
remote networks. All traffic except the needed tunnel traffic should hide
behind the basically all networks except the network
should hide behind the PAT on Site A.

Configuring Site A wrt the NAT:
If you don't have control over the NAT (like with an ISP), you can turn
on "Enable NAT-T transversal" on Site A. With this enabled, you will
have innacurate monitoring of the Site B interface that's NAT'd. It will
report the tunnel is down alot, when it's still up. But, the tunnel will work

I don't believe that leaving the PAT on Site A will work. For one thing, Site
A is not going to be able to talk to the network until the
tunnel is built. And, the ACLs/policies are validated first...before Phase 1
IKE even starts. So, you need to have Site B's public IP...or use the first
hop (outbound)for Site B. Make sure Site B Admin is allowing the tunnel
traffic to pass through the router/gateway that leads to the
network. UDP 500, UDP 4500, TCP 500, TCP 10000, UDP 10000, IP51, IP47

Hope this helps.

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