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New Member

Is NAT to blame.

Here is my scenario.

ASA acting as a VPN server for incomgin client connections.  The ASA outside interface has a 192.168.8.X address.  Infront of the ASA is a internet connected firewall that has a one to one NAT mapping from a public routable IP to the ASA's outside interface.  I have no visibility into the firewall doing the NAT but they tell me they have the relevent IPsec ports allowed.

The client fails to connect and I see virtually no traffic on the ASA for the connection attempt.  Assuming the right ports are allowed NAT is the most likely cause for this failure?  Can someone give some detail on how NAT would be breaking it.


  • VPN
Cisco Employee

Is NAT to blame.


Make sure the allow UDP 4500 In addition to just UDP 500 and IP protocol 50.  When either end is behind a NAT device (client or headend) UDP 4500 starts getting used once both ends realize one of them is behind NAT.

Other than that, make sure that you have 'crypto isakmp nat-traversal' enabled.  You show see it in the config if you do a 'show run all crypto isakmp nat-traversal'.


New Member

Is NAT to blame.


So are you saying that I can connect a client VPN to an ASA with a non public IP provided another device infront of it e.g. a Juniper\Checkpoint firewall etc is doing one to one NAT for that ASA to a public IP?

I thought you couldnt do that or is the UDP 4500 the NAT traversal thats taking care of this?

Cisco Employee

Is NAT to blame.

Yes, you can do that if you're doing 1 to 1 nat.  NAT-T will detect it and encapsulate the IPSEC traffic in UDP4500 which will allow it to be natted.

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