ip cef

int f0/0
ip nat inside
int f0/1
ip nat inside
int s0/0
desc ### 64K ###
backup load 90 5
backup interface serial0/1
ip nat outside
int s0/1
desc ### 256K ###
ip nat outside

route-map ISP1
macth interface s0/0
route-map ISP2
match interface s0/1

ip nat inside route-map ISP1 interface s0/0 overload
ip nat inside route-map ISP2 interface s0/1 overload

ip route 0.0.0.0 0.0.0.0 10.0.0.1
ip route 0.0.0.0 0.0.0.0 10.0.1.1
ip route 0.0.0.0 0.0.0.0 10.0.1.2
ip route 0.0.0.0 0.0.0.0 10.0.1.3
ip route 0.0.0.0 0.0.0.0 10.0.1.4
ip route 10.0.0.0 255.255.255.0 serial0/0
ip route 10.0.1.0 255.255.255.0 serial0/1

я бы сделал вот так...