multiple redundant ISDNs 1

[Foxy]

We have 2 separate ISDN connections to different ISPs. I'm looking for a way to run a mailserver inside my firewall (NT4 network) and have both inbound and outbound mail find its way through even when one or other of the ISDN links fails.<br>
How do I do this?

 

[Salvalinus]

My first reaction was that you would need to do BGP peering with your ISPs, however I think there might be a much simpler solution if all you are concerned about is email.<br>
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First, I assume that you have one mail server (actually this should work with multiple mail servers as well) and that you have been given static IP address blocks from both ISPs. If this is the case then what you should be able to do is assign two IP addresses to your mail server one from each of the blocks assigned by your two ISPs. Then add MX (mail exchanger) records to your DNS tables with one for each of the the two addresses assigned to your mail server. If you assign the same priority to both MX records your mail will normally be delivered over the first record listed, however if that mail server is unavailable (i.e. that ISP link is down) the mail server trying to send you mail should try to use the next available MX entry.<br>
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I haven't thought about this much, so maybe someone will find a major flaw in what I have said, but I suspect this should work for what you are trying to do.<br>
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Good Luck.