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Cisco ASA: NAT exemptions required for VPNs?

Posted on 2009-05-14
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I have two Cisco ASA's, that were originally built by different people.  On asa1, when I build a new VPN (site to site or remote access), I need to go to Firewall | NAT Rules, and add a "NAT Exempt rule".  The source is my networks, destination is the remote networks, and action is "NAT Exempt outbound traffic ..."

However, on asa2, there are no NAT rules, and VPNs can be built (and work) without adding NAT Exempt rules.

Both have identical interface layouts, with the internal interface having a 100 security level, and the public 0.  

My question is, what do the NAT exempt rules do, and why are they required on one ASA and not the other?  Thanks!
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Question by:shanepresley
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tvman_od earned 500 total points
ID: 24385679
The answer is in the processing sequience.
Cisco will apply IPSEC before NAT. So you need to instruct NAT not to touch packets for VPN

Full explanations are here:
http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080133ddd.shtml 
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by:shanepresley
ID: 24394867
Thanks tvman_od,

But could you help me understand why the NAT exemptions are required on one asa, but not the other?  Is there some global config option to bypass the need for NAT exemptions?  on asa2 there are no NAT rules defined, but all VPNs created work perfectly.  On asa1, I need to manually define NAT exemptions to get the same VPNs to work.
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by:tvman_od
ID: 24438057
If you would post both configs I would be able to come up with better explanations. There is a chance that on asa2 you don't have "Split" tunneling and don't have active NAT at all. So all packets are being forwarded into the tunnel and remote peer is visible ising static not-default route. Or something else.
Just remove public and private IPs and put some bogus figures. remove keys and passwords as well.
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by:shanepresley
ID: 24485007
Unfortunately I am not able to post the configs (against our policy).  But thanks for the assistance!
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by:tvman_od
ID: 24488924
You are welcome
If you really need to find out the answer and cannot post configs, you can send me a message over email. You can find address in my profile.
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