What version of hardware/software do you have on the Pix?
The 6500 will errdisable a port that it detects a duplex mismatch on. Try locking both ports down as suggested.
What does the logging buffer say the reason for the disable was?
Check for a bad cable. Also you can use the undocumented command "set option errport enable". This will prevent the switch from disabling that port while you troubleshoot what the issue is. This affects the switch globally and should therefore only be used temporarily to troubleshoot the problem.
Does the Pix's interface work on other switches in 100/full duplex mode?
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...