SAN switch design considerations

Do you need a large number of ISLs in order to build a single FC fabric? This user suggests probably not.

SAN switch design considerations
Dave Reeve

When designing your SAN, think about what you want to achieve. Do you really need any-to-any connectivity, or is your goal just storage consolidation?

Many SAN designs for 16-port switches waste switch ports with large numbers of inter switch links (ISLs) in order to build a single fabric. This provides any-to-any port connectivity but is there any point in doing this?

The answer is definitely "no" if you are looking at attaching consolidated storage products to servers and probably "no" for most SAN configurations. The main function of switches and directors in a SAN is to act as concentrators, and allow a large number of servers to share a small number of paths down to the storage. The SAN fabric inherently provides a server-to-server connection, but there is no function or software available at the present time that can exploit it. Therefore, it makes no sense to provide multiple ISLs between switches just for the sake of it.

Single ISLs are useful in switch environments for distributing zoning information across the switches but should be avoided otherwise.

This was first published in October 2001

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