I have cable internet now but it goes offline from time to time during the day. I work from home so that is a problem. I can get DSL as well.
What hardware do I need to add a backup internet connection or load balancing between the two connections?
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Minimal setup: None. Just set up the second source on the network, plug it into its own router (Or the same one if it supports Dual WAN), and plug both routers into the hub. Set all your computers with a secondary gateway/dns/wins/etc of the second router's IP.
Or you could just buy a router that supports dual WAN, but they're pricey. It'll failover automatically.
As already stated the easiest solution is to use a router that allows for dual WAN connections. I'm currently using the HotBrick LB2 firewall on my (home) network. It's great and faily easy to set up. So far it's been up for about 2 months with no issues.
Some of the newer home routers -- such as the DIR-825 from DLink -- have a USB port on the back where you can plug in a 3G wireless broadband adapter.
Currently using this at home with Sprint EVDO RevA ($59/mo) as a backup WAN.
I've used dozens (literally!) of Draytek 2910s and 2820s with a dual WAN configuration. They are very easy to set up, work very well and are reasonably priced. The 2910 has two ethernet ports while the 2820 has WAN1 as one ADSL port (i.e. plugs straight into the phone line) and WAN2 as an ethernet port. The 3300 that Martin mentioned is even better, but it's two or three times the price.
The Drayteks can use either ping detection or arp detection to detect line failure. If you're using a router on the WAN side of the Draytek you need ping detection as arp detection effectively just tells you that the link light is on. With a cable modem or an ADSL modem like the Vigor 100 you can use arp detection.
you probably want to configure the router to use only one port for HTTPS. In the UK at least a lot of banking sites will panic if they see requests for the same account coming from different IP addresses. The end result is that you are continuously prompted to log in again. On the Drayteks you do this with a load balancing rule.
err, that's it :-)
Example backup routing command for Windows XP/Vista where x.x.x.x is the IP address of your secondary router. We use this in a login script in my office so that if our primary line fails then everything gets routed over our backup link.
ROUTE ADD 0.0.0.0 MASK 0.0.0.0 x.x.x.x METRIC 99
The secondary one should have DHCP turned off. You also want to set your DNS to an external server, or have primary/secondary DNS set to each modem.
I agree with Martin. The DrayTek devices are rock solid and can be found for just a little more money than the less reliable SOHO devices. They even sell devices with the ADSL modem built in which will keep you from buying or renting one from the provider.
Additionally, as Portman stated, you can find routers that will fail over to a 3G wireless adapter. These are great if you travel much because you're able to bring your backup connection with you. Look up the DrayTek Vigor 2910 series routers as they can accomplish both of these options. This is definitely my preferred route.