NAT with two outside interfaces

If you are connected to two ISPs and looking to use NAT, you might have discovered that with the ip nat inside source command, you can only specify one outgoing interface. Since you have two outgoing interfaces, you’ll need to use a route-map to get this working. I will show you how to do this using the following topology:

Above, we have a ‘host’ router that will be our client on the internal network. NAT is our NAT/PAT router, and on the right, we have two ISPs.

Configuration

Let’s configure the host first:

Host(config)#no ip routing 
Host(config)#ip default-gateway 192.168.12.2

First, I will disable ip routing, so it becomes an ordinary host device. We’ll configure the NAT router as the default gateway. Now we can configure the NAT router:

NAT(config)#ip route 0.0.0.0 0.0.0.0 192.168.23.3
NAT(config)#ip route 0.0.0.0 0.0.0.0 192.168.24.4

I will create two equal static routes, one for ISP1 and another for ISP2. This allows us to do load balancing.

These two static routes will allow us to perform load-balancing:

NAT#show ip route static 
S*   0.0.0.0/0 [1/0] via 192.168.24.4
               [1/0] via 192.168.23.3

With our routing operational, we can continue to configure NAT. First, I’ll configure the correct inside and outside interfaces:

NAT(config)#interface fastEthernet 0/0
NAT(config-if)#ip nat inside

NAT(config)#interface fastEthernet 0/1
NAT(config-if)#ip nat outside            

NAT(config)#interface fastEthernet 1/0
NAT(config-if)#ip nat outside

The next step is configuring an access-list to determine what hosts should be NATed. I’ll make sure that the entire 192.168.12.0/24 will be translated: