2 External IP's to one internal using multiple routers/carriers

Unanswered Question
Feb 25th, 2009

I have recently setup a second Inet connection to multiple carriers, goes out Carrier A and the new IP range of goes out carrier B.

We are migrating certain business applications to the new IP address space. In order to avoid any interruption in service I need to be able to provide the service on both old and new IP's concurrently.

I have the Old IP range comfing through carrier A and into a PIX.

The new IP range is coming through carrier B and into an ASA (5540).

I am running HSRP between the routers for failover, both routers are in the same BGP AS, and internet routing is working correctly for return traffic.

I currently have a NAT statement on the Old IP router, translating anything that comes for the old IP to the new IP.

The problem is I can only have 1 default gateway on my ASA, so if I set the Old IP router as the default gateway, the NAT translation works great but anyone who connects to the new IP address doesn't get a connection (the NAT translates it to the old IP going back out to the user)

If I set the new IP router as the default gateway, users can connect to the new IP, but the Old IP comes in, but doesn't get translated going back out.

Anyone know of a solution that could accomplish this?

Thanks in advance,


I have this problem too.
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naveen_b81 Thu, 02/26/2009 - 00:07


Can you give a brief network diagram to understand the issue better?



xcz504d1114 Thu, 02/26/2009 - 08:17

Absolutely, the diagram shows the connection from my AS (AS 1) to the 2 carriers, the 9.x.x.x address range would be the old IP and the 10.x.x.x would be the new IP.

9.x.x.x (old IP) would traverse teh router on the right, and 10.x.x.x (new IP) would traverse the router on the left.

Connections to the old ip address of would come in through the router on the right and need to be sent to the new IP of, NAT translation is the only way I know how to accomplish this. The problem is not delivering the packets to my devices, it is the return traffic, I can only send to one default gateway, if the default gateway is the "New IP Router (10.x.x.x on the left)" then I loose the NAT translation back to a 9.x.x.x address, but the new IP of works fine. If the default gateway is the "Old IP Router" (9.x.x.x on the right) then I can translate back to the old IP for users that requested the old IP, but users that made the request on the new IP will also get translated.

Let me know if you need it in another format other than visio and I'll upload a .bmp file


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