Hello,
below is a basic sample configuration for NAT load balancing using two static default routes. In the example, you have static IP addresses configured on your ISP linked interfaces. If you don't (that is, if you use DHCP, or PPP negotiation), replace the section:
ip nat pool ISP1_POOL 100.100.100.2 100.100.100.2 prefix-length 30
ip nat pool ISP2_POOL 200.200.200.2 200.200.200.2 prefix-length 30
!
ip nat inside source route-map ISP1 pool ISP1_POOL overload
ip nat inside source route-map ISP2 pool ISP2_POOL overload
with
ip nat inside source route-map ISP1 interface FastEthernet0/0 overload
ip nat inside source route-map ISP2 interface FastEthernet0/1 overload
----------------
ip cef
!
interface FastEthernet0/0
description ISP1 Link
ip address 100.100.100.2 255.255.252
no ip redirects
no ip unreachables
ip nat outside
ip virtual-reassembly in
!
interface FastEthernet0/1
description ISP2 Link
ip address 200.200.200.2 255.255.255.252
no ip redirects
no ip unreachables
ip nat outside
ip virtual-reassembly in
!
interface GigabitEthernet0/0
description LAN Interface
ip address 192.168.1.1 255.255.255.0
ip nat inside
!
ip route 0.0.0.0 0.0.0.0 100.100.100.1
ip route 0.0.0.0 0.0.0.0 200.200.200.1
!
ip nat pool ISP1_POOL 100.100.100.2 100.100.100.2 prefix-length 30
ip nat pool ISP2_POOL 200.200.200.2 200.200.200.2 prefix-length 30
!
ip nat inside source route-map ISP1 pool ISP1_POOL overload
ip nat inside source route-map ISP2 pool ISP2_POOL overload
!
route-map ISP1 permit 10
match ip address NAT_ACL
match interface FastEthernet0/0
!
route-map ISP2 permit 10
match ip address NAT_ACL
match interface FastEthernet0/1
!
ip access-list extended NAT_ACL
permit ip 192.168.1.0 0.0.0.255 any
