IPSEC behind a PAT devices

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Oct 23rd, 2008
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Our firewall has multiple site-to-site VPN's as well as it supports Remote Access VPN (using an ASA). A number of RA users who are coming behind a PAT'd address are unable to VPN in, after doing some research I am seeing that a line needs to be added on both firewalls, ie:

isakmp nat-traversal 20

I fear though that this will "hurt" the site-to-site VPN based on this document:


section : Enabling IPsec over NAT-T

Is there any problem enabling this command on the firewall without harming any of the site-to-site VPN's or even RA VPN's?

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ajagadee Thu, 10/23/2008 - 09:15
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You are on the right path, "isakmp nat-traversal" should overcome the issues you are running with RA Users through PAT Device.

And Technically, by enabling "isakmp nat-traversal", you should not run into any issues with L2L Tunnels. Atleast, that is what I have seen with Cisco VPN Servers. In case, if you have L2L Tunnels configured to third party vendors, I would recommend that you enable this command during a "Maintenance Window" and if possible, clear the isakmp and ipsec sa's, and re-establish the tunnels. So, you know for sure that enabling the command did not break the L2L Tunnel Configuration/behavior.



*Pls rate if it helps*

cisco24x7 Thu, 10/23/2008 - 15:50
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If you are doing L2L VPN between Cisco and

and Checkpoint and/or Juniper devices, be

sure to enter this command as well:

no crypto ipsec nat-transparency udp-encapsulation

that will ensure the the L2L VPN to use ESP

instead of sending udp/4500.


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