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[NATTING] Two Internet Gateways (ADSL) and (DIA)

Hi forum, 

 I have one router 2921 edge router connected with two internet connections:

  • Internet connection (1): ADSL. (Dynamic IP) [ Intended to be preferred over the DIA ]
  • Internet connection (2): DIA. (Public IP) [ intended to be a backup for the ADSL & to provide accessibility to a server in my network from the customers ].

 The objective is to route the internet traffic which originated from the LAN to the ADSL router and in case the ADSL failed the internet will be routed to the DIA.

 I have succeed to accomplish the above objective through simulation (GNS 3) by using Static default route + IP SLA + Tracking object.   


 Until i have interrupted with a new challenge which is as following:

 How i should configure the Natting in my router against the DIA and ADSL routers considering allowing any traffic originated from outside the network to reach my server. 

Everyone's tags (1)



When you are using your primary ISP, you should not have any issue because static NAT translates your primary public IP addresses to your private IP addresses. Problem occurs when your primary link fails and traffic is forwarded to your secondary link. In this situation, your router does not translate anymore. 

You can use EEM script to remove your static NAT and configure new NAT statements in case of failure.

Of course, I can have better insight, if you share your configuration.


Hall of Fame Master

I agree with Masoud that if

I agree with Masoud that if we knew more about the environment and the configuration then we might be able to give better answers.

The original post talks about setting up a preferred path and setting up failover using static default route and IP SLA. This seems to be a very appropriate solution. I can not tell from the original post whether address translation has been configured for this or if the configuration so far addresses only the routing and failover. The address translation for traffic originating from the LAN should be fairly straightforward. Since it needs to handle translation for traffic going out two interfaces the address translation should use a route map and in the route map it will match an interface and match the access list which specifies the source addresses. That should take care of the address translation as traffic shifts from the preferred outbound path to the backup.

Masoud  correctly points out that a static translation can be provided for Internet traffic to the server arriving on the DIA interface. However providing failover for traffic from the Internet to the server is quite a challenge. Part of the challenge will be address translation when the interface address is dynamic. Perhaps the bigger challenge will be how to redirect Internet traffic to the server so that if the DIA circuit fails that the traffic goes to the ADSL interface.



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Thanks i have placed the

Thanks i have placed the topology above i appreciate your feedback


As per the above topology.

As per the above topology.

i have preferred ADSL router for host's to reach the internet and GO_DIA as backup using static default router + AD manipulation + IP SLA + Tracking object.

regarding the natting configuration, do i have to configure nating (Overload) on edge router interface fa1/0, considering the ADSL router might configured with natting already. and when the internet traffic fail to GO_DIA i think i have to implement natting overloading for the host and static natting for the server...

am i on the right way? please, correct me if i am wrong..


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