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New Member

Multihomed NAT

Hi peoples,

I am hoping that this is something someone has done before, I’m sure that I cannot be the only one out there wanting this, but I have searched through whitepapers and guides in looking for some help in my current problem.

How do I create a multihomed NAT ?

I have tried to setup 2 NAT pools, but when I shutdown the routes to the primary interface it doesn’t fall over to the “backup” route.

I have read that the reason it doesn’t actually fall over to the secondary pool is due to NAT needing to flush the translations on the interface and the only way to do this is to have a interface down.

Would using one of the new 12.3T tracking features help with this ?

Another method that has been suggested is NAT on a stick, but I have been unable to find a simple guide on how to configure this.

We have a 1801 router, our primary ISP link is a 4meg/4meg connection this is a Ethernet handoff which goes direct into FA0. We also have a backup ADSL link which is supposed to be used when our primary link goes down. Whilst this doesn’t happen all the time, the use of VOIP within our network means that we now need connectivity to the internet at all times.

Unfortunately neither ISP will allow us to do any kind of BGP without us paying a fortune.

If someone could give me some hints / howto guides, I would really appreciate it.

Regards Richard


Re: Multihomed NAT

Use nat with route maps.

Something like this

route-map ISP1

match interface Fa0/0

route-map ISP2

match interface

ip nat inside source list route-map ISP1 interface Lo0 (ISP 1 IP) overload

ip nat inside source list route-map ISP2 interface Lo1 (ISP 2 IP) overload

Then apply nat inside on your LAN and nat outside on the o/gin i/f. Caution :- As yours is ethernet link, you may have to shut the primary ethernet.

New Member

Re: Multihomed NAT

This is the problem that we have.

Stealing comments from "Vivek Baveja"

Once the packet is Nat’ed. It is routed based on Routing Information Base(RIB). And if the interface goes down, the ip route tied to that interface will be withdrawn from routing table and hence no more nat will happen across that interface.

So the interface needs to be classified as down for it to change to the backup NAT interface.

Is it possible to use something like object tracking to be able to automatically switch the NAT external interface


Re: Multihomed NAT

Object tracking can only poll interfaces and perform action based on those. You can integrate it with the route maps i showed above and check if it works.


New Member

Re: Multihomed NAT

Hi Richard,

Here is a guide about Nat-on-stick.



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