Jack is correct that GRE does not provide much security. What you transmit is slightly obscured because the source and destination addresses are the router addresses not your address or the real destination. But the content of the message is clear text, which can be sniffed etc.
If you are concerned about securing the communication through the Internet then you should consider something like IPSec. It is possible to run IPSec between the two PIX or between the two 1841s. I would probably suggest running it between the 1841s. It is easy to combine GRE with IPSec tunnels. I have implemented this a number of times and it works quite well.
The atchitecture that you have described with the 1841 on the outside of the PIX is a good architecture. The 1841 provides routing, tunneling, and protection of the traffic passing through the Internet. The PIX provides various firewall functions which protect the interior of your network. These services may include NAT, stateful inspection of traffic, and implementation of various security policies that your organization may want to implement.
The PIX is a good firewall. The 1841 is a good router. You should let the router route (and tunnel) and let the PIX provide firewall services. Either box is capable of providing both services. In this architecture each box does what it is best at.
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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...