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Hubs, switches and director differences

I would appreciate a clarification as to the differences (either functionally or structure wise) between hubs, switches and directors, in the data center/SAN world. I have a Gartner report that divides the market of the distribution of FC SAN interconnect ports between these three (they also distinguish between entry hubs, managed hubs, and access hubs) and I don't understand the differences. By the way, the definitions in this site do not provide that information. Thanks.

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No comment on Gartner's breakdown except this: just because Gartner identifies a class of products doesn't mean that it defines the industry or the market.

Hubs are shared media interconnect devices that connect nodes to FC loops. They only work with devices that function as loop devices. Managed hubs are those that have an Ethernet interface that might help an administrator do certain things, like disable malfunctioning ports. An unmanaged hub simply forwards FC frames to the next port with something plugged into it. It is not necessary to have a hub at all to form a loop, but it might be crazy to not have a hub if you have more than two nodes communicating.

Switches and directors are based on the same switching technology where every node has a dedicated connection to the network and where the switch connects the end nodes as needed. Directors are high-end switches that have more ports and (hopefully) better fault tolerant characteristics. The distinction is more qualitative than technological.

Hubs and switches can work together. The standards based way for doing it is called an FL port in a switch. This establishes what is called a public loop connection. It is possible to have devices in the loop that cannot communicate through the switch - these are known as private loop devices. A loop can only have a single active FL port connected to it, although redundant standby FL ports are allowed. Non-standard, proprietary mechanisms also exist to connect hubs to switches. These are typically known as "private loop" solutions.

Probably more than what you asked for but sometimes a little extra info can help.

Regards,
Marc Farley

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This was first published in October 2001

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