Storage Area Networks For Dummies®

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Par t I: SAN 101
Basic SAN port modes of operation
The port’s mode of operation depends on what’s connected to the other side
of the port. Here are two general examples:
✓ All hosts (servers) and all storage ports operate as nodes (that is, places
where the data either originates or ends up), so their ports are called
N_Ports (node ports).
✓ All hub ports operate as loops (that is, places where the data travels in a
small Fibre Channel loop), so they’re called L_Ports (loop ports).
Switch ports are where it gets tricky. That’s because switch ports have multiple
personalities: They become particular types of ports depending on what
gets plugged into them (check out Table 2-2 to keep all these confusing port
types straight). Here are some ways a switch port changes its function to
match what’s connected to it:
✓ Switch ports usually hang around as G_Ports (global ports) when nothing is
plugged into them. A G_Port doesn’t get a mode of operation until
something is plugged into it.
✓ If you plug a host into a switch port, it becomes an F_Port (fabric port).
The same thing happens if you plug in a storage array that’s running the
Fibre Channel-Switched (FC-SW) Protocol (more about this protocol in
the next section).
✓ If you plug a hub into a switch port, you get an FL_Port (fabric-to-loop
port); hub ports by themselves are always L_Ports (loop ports).
✓ When two switch ports are connected together, they become their own
small fabric, known as an E_Port (switch-to-switch expansion port) or a
T_Port ( Trunk port).
✓ A host port is always an N_Port (node port) — unless it’s attached to a
hub, in which case it’s an NL_port (node-to-loop port).
✓ A storage port, like a host port, is always an N_Port — unless it’s
connected to a hub, in which case it’s an NL_Port.
If that seems confusing, it used to be worse. Believe it or not, different switch
vendors used to name their ports differently, which confused everyone. Then
the Storage Network Industry Association (SNIA) came to save the day and
standardized the names you see in Figure 2-19.
If you want to get a good working handle on what’s going on in your SAN, use
Table 2-2 to find out what the port names mean after all the plugging-in is done.