Since many-to-one address translation is common with many environments where remote-access clients are deployed, a special mechanism called NAT transparency exists to overcome these NAT issues. NAT transparency NAT-t re-encapsulates the IKE and ESP packets into another transport layer protocol, such as UDP or TCP, which enables address-translating devices to perform translation correctly. Learn more about NAT-t here.
Also, here's a very good article by Lisa Phifer on NAT & IPSEC issues.
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