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Backup Line

Just wonder if anyone can point me to the right the direction, I would like to have the backup line for my network. I plan to go with either DSL or ISDN with different ISP. Assume that I have 2 Internet lines go with ISPA (Primary) and ISPB (Backup). I also host Web and Mail server in my network. I want to minimize the down time as much as possible. The problem is when the failover occurs (ISPA down), I can go out to Internet by ISPB network but people on the world can not hit my Web and Mail server because the DNS entry for Web and Mail server still point to the ISPA Network. I know that you can change your DNS entry point to the ISP B network but it will take some time to populate it. Anyone knows others way around or any advice on this would greatly appreciate.



Re: Backup Line

The normal way to achieve inbound redundancy is to get an Autonomous System Number and use BGP with both ISPs, but finding a DSL ISP which supports BGP can be difficult. You are already aware that DNS can do the job, but only if you are willing to put up with caches not being flushed and defective host implementations. Some ISP's will work with you to either use run a routing protocol with the other ISP or advertise your IP block using their ASN, but you're really playing games with routing protocols and many ISPs refuse to play the game.

Have you considered outsourcing your servers to a provider which has adequate connectivity? Or installing a redundant connection with adequate physical diversity to another POP of the first ISP? Both of these approaches are far more likely to buy you more availability for the $$ than a hacked up solution with two ISPs (you'll find the link between you and the ISP is far more unreliable than the ISP, if not, find an ISP that knows what they're doing).

For a general overview of multihoming approaches, with and without BGP, see the white paper on my web site.

Good luck and have fun!

Vincent C Jones