Dual-homed environments with different ISPs have usually meant that BGP must be used and that both ISPs must support BGP routing. Throughout this tutorial, I will show you how you can have mulitple ISPs without BGP or provider independant IP addresses. Tracking objects provide a means of removing static routes if the ISP cannot be contacted.

Here is a small drawing: Dual Internet with NAT

hostname Router
!

ip cef

!####Establish sla monitors for use in tracking objects####!

ip sla monitor 1
  type echo protocol ipIcmpEcho 12.34.45.1
  threshold 3
  frequency 5

ip sla monitor schedule 1 life forever start-time now

ip sla monitor 2
  type echo protocol ipIcmpEcho 23.34.56.1
  threshold 3
  frequency 5

ip sla monitor schedule 2 life forever start-time now

!

!####Configure Tracking objects (referencing IP SLA monitor’s above)####!

track 101 rtr 1 reachability
!
track 102 rtr 2 reachability
!
!
!
!
!####Configure Interfaces with NAT####!

interface FastEthernet0
  ip address 192.168.1.254 255.255.255.0
  ip nat inside
!
interface s0/0
  ip address 12.34.45.2 255.255.255.0
  ip nat outside
!
interface s0/1
  ip address 23.45.67.2 255.255.255.0
  ip nat outside
!
ip classless

!####Configure gateway of last resort with tracking objects####!
ip route 0.0.0.0 0.0.0.0 12.34.45.1 track 101
ip route 0.0.0.0 0.0.0.0 23.45.67.1 track 102

!####Configure NAT statements for most outbound traffic####!
ip nat inside source route-map ISP1 interface s0/0 overload
ip nat inside source route-map ISP2 interface s0/1 overload

!####Configure NAT statements for your mail server####!
!(remember to setup dns for mail on both public IP addresses)!

ip nat inside source static tcp 192.168.1.10 25 12.34.45.2 25 route-map ISP1 extendable
ip nat inside source static tcp 192.168.1.10 25 23.45.67.2 25 route-map ISP2 extendable
!
!
access-list 10 permit 192.168.1.0 0.0.0.255
!
!####Configure route maps for reference in NAT statements####!

route-map ISP2 permit 10
  match ip address 10
  match interface s0/1

!
route-map ISP1 permit 10
  match ip address 10
  match interface s0/0
!

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