NAT with two outside interfaces

Lesson Contents

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:

NAT Two Outside Interfaces

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.


Let’s configure the host first:

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

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
NAT(config)#ip route

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

To make your default routes reliable, I recommend you configure object tracking and IP SLA.

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

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

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 will be translated:

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Forum Replies

  1. Thank you for this awesome post… keep it up!

  2. Thank you, this site is awesome.

  3. Hi bhargavi,

    In our lessons, we used route-map with different routing protocols for routing redistribution and/or filtering. I haven’t found a specific chapter explaining specifically about route-map in our website, but you can see how Rene has used it in different routing protocols on the following 2 lessons:

    EIGRP Route-map Filtering
    OSPF LSA Type 5 Filtering on Cisco IOS

    I can refer you also to this article from Cisco website where you can find all information needed about route-map:

    Route-map explanation from

    Hope this can help.

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