ISP changeover/NAT and routing

Unanswered Question
Jul 23rd, 2010

I wonder if anyone can assist:

We are changing ISP, and for a short period have two lines, each with a 2811.   We use NAT translating a number of IP addresses onto one internally addressed web server. Unfortunatly I can't transition all the sites to the new IP address range at the same time. This means I can't change the gateway address an the web server without cutting off half the web sites.

On the old ISP at least, BGP is out of the question.

It has been suggested that traffic coming down the old ISP line could be tunneled to the new router, but then default routes don't allow for the return route through the tunnel as far as I can see/

Is there a way to allow for NAT traffic via two lines and two routers to one server?   I wonder if somhow nat outside can be used.

I'm sure this must be a fairly common issue!  What is the best way to solve it?

I have this problem too.
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Nagaraja Thanthry Fri, 07/23/2010 - 08:02


Do you have control over both routers? If you do, then you can configure

route-maps and nat overloading such that all requests coming from the old

ISP seems like is coming from the routers inside interface. This way, when

the webserver is serving pages for those requests, it will send the response

back to the ISP1 router which is on the same subnet and not worry about the

default gateway. However, one change that you do need to do is configure NAT

on a virtual interface on the old ISP router.

Step 1:

Remove all existing NAT rules

Step 2:

On both outside and inside interface, enable NAT

Interface fa 0/0

ip nat enable


Interface fa 0/1

ip nat enable


Step 3:

Reconfigure all NAT rules

Step 4:

Configure an access-list matching http traffic destined to your webserver

Access-list 199 permit tcp any host overload

This should address your issue.

Hope this helps.



timhavenhand Fri, 07/23/2010 - 08:09

Thanks, that looks very helpful, though I'm not entirely sure I follow the bit about configuring NAT on a virtual interface.  But seems like that is what I should invesigate.




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