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We have a small office network connected to the Internet via a WiFi router and cable modem. 20 or so machines, some wired via a switch to the router, some wireless. We also have a standalone machine in the office connected to the Internet via a separate WiFi router and ADSL modem. The two networks aren't currently connected in any way.

What's the simplest way for us to use the ADSL Internet connection as a backup for our cable modem connection should it ever go down?

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2 Answers 2

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The absolute simplest way is to have the ADSL modem running at all times, and then unplug the cable modem from the Wifi router when the connection goes down, and connect the ADSL modem in its place.

This is silly though. Requires manual intervention etc.

You could have a device that loadbalances your connections, The Netgear FVX 538 can loadbalance across two WAN links, but I don't personally recommend it.

You could do some very clever things with a Cisco router to manage both connections instead. You do however have the problem that your external IP changes when you switch connections. With a small network setup, there's not a lot you can do about this, but with the correct providers and cooperation, it is sometimes possible to have seamless failover and not change IP when changing provider.

You could probably do everything quite cheaply using a computer as a firewall/router device with a custom linux distribution like pfSense or similar.

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not all cisco router support multiwan –  mezgani Dec 7 '09 at 11:06
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No, but there's over a thousand different router configurations and models, and I haven't got space to list all the ones that do and don't here. Perhaps googling "cisco multi wan router" might be a good starting point. –  Tom O'Connor Dec 7 '09 at 11:34
    
Thanks. Seems like unplugging is the easiest and cheapest option then. It's not something we need to spend money on, I just wondered if there was some trick I was missing. –  John Francis Dec 9 '09 at 13:32
    
It's infuriating, but this is actually the simplest solution. If you find that's not working for you, perhaps a dual-wan router is for you. Time will tell. –  Tom O'Connor Dec 9 '09 at 14:17

Tom is right about the range of options - Cisco may be overkill for most small offices, and Netgear may be more on the home-user end of the spectrum.

Most midrange firewalls should be able to handle this. Sonicwall and Firebox are a couple of brands that will have a few options for multi-WAN with failover.

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I didn't really want to specify Cisco models, but I will say that for most small offices, an 1800 series would probably offer the right feature sets. It's actually non-trivial to specify the exact solution. I could do it, but it'd take more information than just this question to make the right choices. One advantage of Cisco routers might be to replace the cable modem with a Cable HWIC on the router backplane. Mucho $$ though. –  Tom O'Connor Dec 7 '09 at 17:12

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