Uttam,
I assume you are refering to upgrading the IOS code for firewall. If so go over this link bello which has excellent information on IOS firewall and provide you with ALL you need to know for inplementing IOS firewall in your platform.
http://www.cisco.com/en/US/products/sw/secursw/ps1018/index.html
IOS Software it can be found here:
http://tools.cisco.com/Support/Fusion/FusionHome.do
1- choose Find Software with features
2- Pick your platform ( Router 2811 )
3- Pick ( Firewall Feature set)
You can compare features between Advanced IP services and Advance Security, any of these can provide Zone based firewall.
Regards
Jorge Rodriguez