By default Catalyst 3550 doesnt support NAC.There are certain IOS Images which supports NAC.NAC - L2 IP is supported by IOS Version 12.2(25)SED.See if this is the version in the switch,if not try upgrading the Image to the one mentioned.Refer the URL
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...