Q

Switches and hubs, do they function the same?

Can I say that a switch and a hub function is actually the same?

You can use a switch OR a hub for connectivity to an external Fibre Channel storage subsystem. When you use a hub for the connection, you will be using the Fibre Channel FC-AL protocol (Fibre Channel arbitrated loop).

Using a hub to connect servers to storage is very easy to set up and works fine for many lower end environments. When you use a hub, everyone shares the same bandwidth to the attached storage. Since the loop is "arbitrated", each server needs to arbitrate for access to the loop to transmit data. Only one device can communicate across the loop at a time.

In a switched environment, every server has point-to-point connectivity through the switch to the attached storage. Using a switch will allow each server to have its own 100MB or 200MB connection through the switch (it's 100MB with 1Gbit Fibre Channel and 200MB with 2Gbit Fibre Channel).

Since each connection is point-to-point, adding servers to a switch environment does not cause any interruptions of data traffic. If you add a server to a hub, a "LIP" occurs (loop initialization) which affects every server connected to the hub. This is why switched fabrics are used more commonly today than hub based solutions. Only 127 devices can be connected in an FC-AL loop, where thousands of devices can be connected to a fabric.

If you have hubs that you would like to re-use, one good method is to add another host bus adapter to the servers and use the loop as a separate backup network for connection to your tape drives. This would free up backup from the fabric and let you re-use your investment in your hubs.

Chris

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This was first published in November 2002

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