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New Member

VPN client/Nat traversal - what ports needed?

I was troubleshooting a cisco VPN client problem today (other end is a 3030 concentrator). Even though NAT traversal was enabled on the client I still had to open up ISAKMP (udp port 500) on a firewall between the two endpoints to get the tunnel to come up. Running some traces show that ISAKMP/IKE exchanges still use udp500.

Is this normal behavior? I thought all communications would use UDP 4500 for NAT traversal but I guess that is only for the ESP packets and not IPsec itself?

Thanks for any insight.


Re: VPN client/Nat traversal - what ports needed?

ISAKMP negotiation needs to occur on udp 500. After both sides agree that they can use nat-t, then commo migrates to udp 4500

New Member

Re: VPN client/Nat traversal - what ports needed?


A VPN connection requires two different kinds of traffic UDP port 500 (used on phase 1) and ESP

protocol (used on phase 2). Even when your site has both of them open for outbound connections, if your user is going our through PAT (NAT-T issue), ESP traffic will be blocked because it has no port information at all.

The solution is to enable one of two features available on the Concentrator. The first one is called NAT Traversal (also called NAT-T) and what is does is encapsulate ESP packets over UDP port 4500.

So you needd to open both the ESP and UDP 4500 ports.

Let me know if you have any queries.