When you need a redundant WAN connection, there are several options and several things you should consider. Here...
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are a few of the popular ones in no particular order:
Leased-line with dial backup. Leased lines are permanent, "always on" solutions like T1s and T3s and such. Dial refers to circuits that connect to a cloud like the PSTN, but usually they're ISDN BRIs or PRIs. This option is usually the cheapest, but performance in a failure will usually stink. Also, ISDN has a bad reputation in many parts of the country, which is probably due to the propensity for administrators to horrifically misconfigure it. The ISDN dial circuit can either go to the same service provider the primary circuit connects to, or you can have it go to one of your sites in a "dial-around-the-cloud" scenario for diversity.
Two leased-lines in an active/passive configuration. This means you have 2 separate circuits, but you only use one until it fails, then traffic automatically switches over to the other. This is usually cost-effective because you can get good deals on the backup circuit, since service providers know you won't be using up their valuable backbone. Also, there's nothing keeping you from making the backup circuit the same size, or smaller.
Two leased-lines inverse multiplexed. This interesting configuration gives you two circuits bonded together so that they appear as a single circuit. The primary advantage to this is that the maximum throughput a single user can achieve is limited by the size of the circuit he's using, regardless of how many circuits there are. This is because most load-balancing schemes are based on "flows" where all packets in a flow follow the same path to prevent out of order packets. With two circuits inverse multiplexed, they appear as one larger circuit, resulting in more throughput for an individual, and when one circuit fails, only that portion of the bandwidth is lost. The downside to this is that both circuits have to connect to the same router, and if it fails, you lose both circuits.
Finally, two leased-lines in an active/active configuration. This load-balances the traffic across both pipes and is the Cadillac of redundancy, but you have to keep an eye on utilization. As it inevitably grows over time, you'll get to a point where both circuits are more than half full, which means if one fails, you'll have serious performance issues.
Tom Lancaster, CCIE# 8829 CNX# 1105, is a consultant with 15 years experience in the networking industry, and co-author of several books on networking, most recently, CCSPTM: Secure PIX and Secure VPN Study Guide published by Sybex.