We are trying to setup a network whereby there are two routers, Router_A and Router_B. Both routers have their own ISP connection, so effectively IPADDRESS_1 and IP_ADDRESS_2.

Behind these routers we have an SBS 2003 server which is setup with to NIC cards in standard configuration (1 nic card for internet access pointing to ROUTER_A and one nic card for the local network).

We are trying to set it up so IPADDRESS_1/exchange and IPADDRESS_2/exchange both work. However currently only IPADDRESS_1/exchange works.

We have setup Router_B to forward all ports (DMZ) to Router_A. We have tried two different makes of router for Router_B however still have had no luck. We have also tried sending the traffic from Router_A and Router_B directly to the server, however this did not work either.

Should this be working? Or are we missing something?

P.S I appreciate that this looks like a scenario for a load balancing router, however we want to protect against the times that a router may fail, in this case the external clients only have to change the IPADDRESS they are connecting to.

  • So you're setting this up as a failover? If so think this is the wrong approach. You need some routing on the windows box. – Dan Jun 7 '11 at 8:56

Please, do yourself (and any other network admin that has to look at this setup in the future) a favor and get a router that's capable of multiple WAN connections: invariably, they'll have some combination of load balancing and failover functionality built-in and will do what you want, without having to mess with routes, NAT policies, etc.

A quality firewall that's free/open source (you just need hardware to run it on) is pfSense. It can do load balancing/failover out-of-the-box, has solid commercial/community support, and is rock solid. I have close to a dozen in production at various clients (several of them multi-WAN) and have not had a single failure or issue.

Also, multihoming your SBS server is not recommended anymore, in fact, starting with SBS 2008, it's an unsupported configuration method.

  • Thank you very much for all your answers. We will go down this route. – Tribal Jun 7 '11 at 13:02

It cannot work. The SBS2003 server has a default gateway pointing at Router_A. Any outgoing packets for the Internet will go to that device, even if they come in via Router_B. Most ISP's will drop packets where source and destination of a return packet do not match the original request (as this can be a sign of a man-in-the-middle attack).

You can set up a second default route with a higher metric than the original one, and this will kick in once the original default route becomes unavailable (because Router_A is down), but as long as Router_A is up, that's the route that will be used. Combined with the relevant DNS settings this can result in a failover setup, but you cannot achieve load balancing in this way. If that is your goal, you will need to use either a proper load balancer or some other solution.


You should be forwarding the traffic that is going to Router_2 to the actual device that needs to receive it.

So all email traffic is going to Router_A in external DNS, or is it going to both?

I'm not quite sure why there are two routers and not just one PIX with one line that has multiple IP's assigned to it?

(I must be missing something)


  • Sorry I should have mentioned that we did also try sending the traffic going to Router_2 to the server itself and this did not work either. – Tribal Jun 7 '11 at 9:19
  • So does Router_1 and Router_2 both have different static IP's or dynamic? Also what type of traffic is going to each router? – 0x0000001E Jun 7 '11 at 10:06

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