The risk of a disaster striking a widespread area will vary based on the geography and the risk being evaluated itself. For example, a distance of 10 miles between two sites in a region prone to tornadoes could be considered sufficient. However, the same distance in hurricane country would be largely inadequate.
The next factor to consider is the likelihood of a catastrophic event taking place. For example, the 2003 power failure in New York reached as far as Toronto in Canada, which is approximately 450 miles away. How many organizations had ever anticipated the fact that both cities were on the same power grid and that the entire power grid was going to fail? The question then becomes: How likely is this to ever happen again?
Unless one has an unlimited IT budget, an organization must decide what is an acceptable risk to its business. The financial impact an event could have on the business and how much it would cost to reduce that impact to an acceptable level will influence the final decision.
To summarize, conducting a risk evaluation and business impact analysis will allow an organization to determine the adequate distance between a main and DR site.
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