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NAT with Two ISP ( External IP from ISP-1)

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Last Modified: 2013-12-14
I have exchange server running on NAT environment.  10.1.1.2 is exchange server internal IP Address and external IP Address is 120.X.X.X.  We have two ISP connections and exchange server external IP address is given by ISP-1. When we connected to our ISP-1 everything is working fine. When the ISP-1 is down and we will connect to Internet using ISP-2.
Since the exchange server external IP is from ISP-1, there won’t be any route through ISP-2 to reach our exchange server. So we could not send/receive any e-mails while we connected to ISP-2.  We are using SONIC firewall and exchange 2007.
We cannot do NAT from two external IP to one internal IP. So taking external IP from ISP-2 and NAT to 10.1.1.2 is not possible. At the same time I don’t want assign another IP (like 10.1.1.3) to exchange server and assign ISP-2 for NAT. I think this option also won’t work.
Please suggest a solution for this.

Thanks
Ram.
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