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?
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.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...