Correct me if I am wrong, but you mentioned that the routers are outside PIX, which they shouldn't have problem to sync with any external NTP Servers, as well as PIX itself. But of course, you'll have problem if you want them to sync with internal NTP Server.
If Routers need to sync with internal NTP Server (internal router, etc), you need to map the internal router with Public IP, or use PIX outside interface to redirect NTP data (tcp/udp-123, pref udp 123). Open ACL and allow ntp protocol only between outside routers and internal NTP server/router.
If routers are on internal network, open ACL and bind to inside interface - allow udp 123)
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...