This basically indicates that frames being looked at are of a feature which is not supported. If you are running IOS 12.1, the reason would be that all the frames to the SPAN destination port come with 802.1Q tag which SNOOP does not support. You could use a sniffer to check the same.
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...
[toc:faq]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 and UDP are I...