The way to make any connection more reliable is with diversity and redundancy. Many enterprise networks have redundant routes with redundant connections to the edge via diverse physical paths. Although such a setup adds cost and complexity, we consider this insurance well worth the trouble. Accomplishing this same duplication on the Internet, while not cheap, is possible by using multiple ISPs to provide redundant and diverse paths from end to end. Although this approach can be difficult and expensive, it is de rigueur for any organization serious about delivering applications reliably over the Internet. The question then becomes whether you develop these diverse paths yourself, outsource connectivity to a mega provider while keeping Web servers in-house, or outsource the whole shebang. There are pros and cons with each tactic.
First, the gotcha: Having multiple ISPs doesn't guarantee true redundancy. For example, what if physical access to both ISPs is provided by the same local carrier, which has only one path into your building? Answer: One backhoe could put an end to your best-laid plans. You need to establish multiple, divergent physical paths into and out of your building. This can be provided via a Sonet service from the carrier, but be aware that Sonet doesn't guarantee diverse physical paths unless it is designed correctly, and it might not be cost effective for you or the carrier.
You also must be clear about your goals. Do you want to load balance to get the most use of both links, or purely provide redundancy? If you load balance your connections among multiple links, and start using more than 50 percent of the capacity of both, you won't have full redundancy: If one link fails, the other won't be able to support the additional traffic. A situation like this could be as bad as having the link go down completely because expectations are raised, and performance becomes unpredictable. If you haven't oversubscribed too much, you can compensate somewhat by prioritizing traffic.
Then there's the bottom line. The cost of having two expensive links each being used at less than 50 percent of their capacity, not to mention all that extra hardware, is considerable. Be prepared to make the case that there is a cost for true redundancy, and decide if there is a business case to justify it. This problem may be compounded by lingering fears that many ISPs are on shaky financial ground. Furthermore, low-speed Internet-access link prices will increase by 25 percent through 2005, according to Gartner. Choosing an established ISP as your main provider can give you some flexibility to use a less-known (and possibly less-expensive) provider as a secondary. Some suggestions to make your case:
Consider entering into a contract of sufficient length so that installation fees will be waived.
In exchange for a long-term contract, which benefits the ISP by reducing churn, ask for a price-stabilization clause.
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