The PIX can't do any load sharing or similar to two different ISP's. You have to be very careful that traffic that goes out a particular interface in the PIX comes back in that same interface, otherwise it'll be dropped. You can only have one default route in the PIX also.
You're best off connecting the two ISP circuits to your outside router and then just create a small subnet in between this router's inside interface and the PIX's outside. Let the router do all the load-balancing between ISP's, etc, and the PIX will just point everything to the router.
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...