VPN Implimentation IPSec/GRE Tunnel using 1720 and 831 Routers

At each site I have ADSL connections to the internet, with static IP
addresses. The Cisco 831 has two ethernet ports in it and so does the 1720,
both routers have the nessasary IOS versions for a IPSec/GRE tunnel. I will
connect each router to the ADSL modem via one of their Ethernet ports and
the second ethernet port will be for the LAN. The connection between the
cisco devices and the adsl will be using nat, im just wondering if this will
be any problem for me or will nat-t do it's magic here ?

 

initially it will be just 2 sites, 1 branch office with the cisco 831, and 1
head end router at hq, a cisco1721. i don't want slpit tunneling at either
end, the tunnel between the sites will be IPsec/GRE.

im concerned that these two cisco devices the 1721 and the 831 may not be
able to do an IPSec tunnel because they are not directly connected to the
internet eg. by an adsl wic, but instead their connection to the internet
will be given by a 3rd party ( nokia ), adsl router. Will having the third
party adsl router cause me problems ?
or do i need to use adsl wic's ?

Thanks, hope this is helpful.
Ali

 

I am a little confused. Are you saying you have many sites that will
have LAN 2 LAN tunnels using GRE/IPSEC and at each site there will be
split tunneling to the internet?