Hi Siva,
If you want to build the IPSec tunnels from your routers and not from the FW, you need NAT-T feature. Also your FW should allow UDP on port 4500 from the outside in addition to allow it from the inside.
It would be easier to build your tunnels between the FW
The following link gives you plenty of examples:
http://www.cisco.com/en/US/tech/tk583/tk372/tech_configuration_examples_list.html
HTH
Laurent.
