how to archive this with vpn?

[henrybb]

lan a--router a---router b----lan b ----router c-----lan c

can lan c communicate wth lan a and lan c CAN'T communicate with lan b?

how to config vpn to archive this without ACL ?

can anybody tell me how to find more info about GRE tunnel ? I can't find it on cisco.com.

thanks.

[tcross3]

Check this link out;

http://www.net.usda.gov/tunnllab.html

Also;

http://www.cisco.com/warp/public/105/56.html

Hope this helps