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Community Member

NAT for 3 ISP connections

I have 3 Internet connections with 3 different ISP's, each with 8 IP addresses.

If all these connections terminated at a single router.

Can I do NAT to map all these 3 different segments of IP addresses to one SINGLE contiguous inside IP addresse range ?

Also, I want to do some sort of traffic filtering to pass different traffic ( based on TCP port number ) to different ISP links.

Community Member

Re: NAT for 3 ISP connections

I would have TAC help you with this. It's a complex config and yes, it should be doable in one way or another.

Community Member

Re: NAT for 3 ISP connections

I having some idea as to how to achieve this by defining NAT using route maps. The point that I cannot figure-out is how can I define multiple default routes in my router. ( I am not running any routing protocols between my router and ISP ).

Can this be achieved without any routing protocol (like BGP ) between ISP and customer ?

Cisco Employee

Re: NAT for 3 ISP connections

to simplest way to add multiple default route is to use static routes.


ip route x1.x1.x1.x1

ip route x2.x2.x2.x2

ip route x3.x3.x3.x3

For nat, route-map seems to be a good solution.

To route to different ISP based on application type (TCP port number) you will have to use policy routing.

But before using it, be aware that policy routing could reduce the performance of the router.

Community Member

Re: NAT for 3 ISP connections

Many thanks for the reply.

In fact, I have configured the multiple routes in the same way as you suggested in my test setup. But it doesn't seems to be working properly. What I noticed is the results cannot be predicted. Sometimes it works right. But sometimes the router just selects only one default gateway from the defined gateways and tries to send all the traffic there.

Any suggestion ?

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