Hi,
One solution is to configure a static NAT on the 871.
ip nat inside source static x.x.x.x y.y.y.y
interface e1 --> inside
ip nat inside
interface e0 --> outside
ip nat outside
So, all the traffic received on the router on public IP y.y.y.y will be sent to the private IP of the ASA x.x.x.x (for example to terminate VPNs)
Federico.