I have the exact same problem. The suggestions I am getting relates to the "increasing the key life time." In fact also have the PIX 515 with the same IOS you are using. Mine is sitting in front of a Cisco VPN Concentrator 3030. When my tunnels drop, I just reboot my PIX and all is fine again, for about 4 more days. I am going to try the increasing of the Key lifetime and see what is does.
Thanks for your reply to my posting. Increasing the key lifetime I am told will make the VPN less secure as the keys are not renegotiated as often. The same problem will occur but it will be over a longer period.
The tunnel will close as soon as the key expires even if the key lifetime is doubled and it will have the same issue renegotiating the key.
I have tried extending the key lifetime on the pix and the remote peer and my theory has proved to be true loss of connection after the key expires and problems renegotiating the tunnel.
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We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...