Two Gatway network (Linux gateway and pix 515E gateway)

i am currently using linux as a roter firewall and gatway of my network.

i am having one pix515E firewall with three interface.

i am nat a public address x.x.216.116 also a transperent proxy server. with out changing how to utlize the firewall.

local net (10.2.10.0/24) ---------> linux box ->out going ip x.x.