I am afraid whether it is possible to set up the VPN failover the way you need with a 506 at this time. It is a possibility for any pix that you can load version 7.0 on. Right now that would be for the 515's and higher. 7.0 software has the ability to re-route traffic on the outside interface. So better, you first verify with this issue.
PIX 506 E supports 25 site 2 site or remote access tunnels at a single point of time. I would request you to kindly upload the config of PIX along with the full debug messages, so that we can have a look at it.
I would suggest that have you applied proper access-list on each site 2 site VPN ISAKMP configuration. If you have 3 site 2 site VPN, make sure that proper access-list is defined so that appropriate traffic is generated and tunnel is formed.
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...