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Force the use of NAT-T on an IPSEC L2L tunnel


Hello folks

This might be an odd question, but can I force an IPSEC L2L tunnel to use NAT-T encapsulation no matter what? Automatic detection says none of the endpoints are behind NAT.

I know I can disable it by the "crypto map XXX set nat-t-disable" command, but I want the exact opposite.

I have a very strange issue where asynchronos routing is making my life as a technician very hard.

A side question; Can I do something about an ISP that is policy-base-routing its ESP traffic (and/or translating it)?

ASA5505 ===>===>===> ISAKMP traffic ===>===>===> ASA5510                                                        (traffic source IP:

ASA5505 ===>===>===> ESP traffic ===>===>===> ASA5510                                                  (Traffic source IP:  - when it should see

The above is meant to show that I have an ASA5510 that is configured with an L2L tunnel with peer

The ISP of is somehow not doing its routing/translating correctly as the source IP of traffic originating from my ASA5505 is another than the one configured on the ASA5505. This is only the case when talking about ESP traffic. UDP (ISAKMP) traffic is correct.

The VPN tunnel is successfully established (both phase 1 and 2), but no traffic can traverse the tunnel. It has been working fine untill this morning.

Not long ago ISAKMP traffic was translated the same way as ESP traffic - it was working then, as long as the non-NAT'ed device initiated the tunnel.

I hope I have explained myself, so that you can understand it

Best regards

-- Jesper

3 Replies 3

Maxim Zimovets

Hello, Jesper!

Well, I can suggest you couple of solutions.

First You can install some other device in front of  ASA5510 and set up NAT on it. In such a case you will forse both your ASA to use NAT-T.

I think it's not practical.

Second solution is as follows. In the old time of Altiga VPN-concentrator it was possible to use IPSec over TCP. Now ASA supports this mode as well. Please, look through following URL - and

I hope this will help you.

With best regards


Hello Maxim

Thank you for your reply. I've all set to try out your suggestion until I stumbled upon this in the documentation:

"It is a client to security appliance feature only. It does not work for LAN-to-LAN connections."

So unfortunately your suggestion is a no-go. :/

Best regards

Jesper Ross.

P.S. The ISP finally got their act together and fixed their routing, so that ISAKMP and ESP was sent using the same IP address.

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