Encrypt IDS traffic between PIX and R4 in Section 6.2.1. Use a preshared key and SHA for message authentication and DES for encryption. Configure all other parameters as you feel appropriate.
Configure IPSec to encrypt GRE traffic between R6 and R3 in Section 5.2.
Use preshared keys. Configure all other parameters as you feel appropriate.
If there is a loss of connectivity between two IPSec peers, terminate the sessions.
You are allowed to put one static route but not a default route on the PIX to achieve this task.
Configure GRE through PIX; R6 should see all loopbacks ...