Storage Area Networks For Dummies®

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Par t I: SAN 101
The Switched Fabric
When you connect two switches in a SAN, you create a switched fabric. SAN
switches tend to be much smarter than SAN hubs because a switched fabric
can contain many more devices than a loop can. Whereas a loop can have a
maximum of 127 devices connected to it, a switched fabric can contain thousands
of devices.
Switches are sometimes called nonblocking devices: That is, each device can
communicate with another device without having to wait its turn. The inside
of a switch is designed so that multiple physical connections exist between
each port and every other port on the switch. (See Figure 3-3.)
Figure 3-3:
A nonblocking
SAN
switch.
Every port has access to every other port.
Multiple conversations can happen through a switch simultaneously. Conversations
are routed between the devices connected to the switch ports using
the FC-SW protocol. If you connect two switches to create a fabric, the protocol
lets the switches discover each other, allowing communication to happen
across switch boundaries. This connection between the switches is an interswitch
link (ISL). By using the FC-SW protocol, any device connected to any
port on any switch can communicate with a device on any other switch port.
For example, the server in Figure 3-4 that is connected to Switch 1 can
communicate with the storage connected to Switch 2 by using the ISL
between the switches.
You can create very large switch fabrics. Thousands of devices can be connected
through switches, and switches can be connected to make very large
fabrics. And because communications between those devices can all happen
at the same time, SAN switch fabrics are very scalable. This is also why SAN
loops are going the way of the dodo. Using a fabric rather than a loop in a
SAN makes more sense for some practical reasons:
✓ A fabric can be designed to allow you to nondisruptively add more
switches to expand the network.
✓ Switches can be added to expand the network without affecting what’s
currently happening in the fabric.
✓ The rest of the switches in the fabric automatically discover each new
switch and any devices connected to it.