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 mul-
tiple 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.
Protocols used in a Fibre Channel SAN
Protocols are, in effect, an agreed-upon set of terms that different computer
devices use to communicate with one another. A protocol can be thought of
as the common language used by different types of networks. You’ll encoun-
ter three basic protocols in the Fibre Channel world:
✓ FC-AL: Fibre Channel-Arbitrated Loop Protocol is used by two devices
communicating within a Fibre Channel loop (created by plugging the
devices into a hub). Fibre Channel loops use hubs for the cable connec-
tions among all the SAN devices. Newer storage arrays that have internal
fiber disks use Fibre Channel loops to connect the disks to the array,
which is why they can have so many disks inside: Each loop can handle
126 disks, and you can have many loops in the array. The array uses the
FC-AL protocol to talk to the disks.
Each of the possible 126 devices on a Fibre Channel loop takes a turn
communicating with another device on the loop. Only one conversa-
tion can occur at a time; the protocol determines who gets to talk when.
Every device connected to the loop gets a loop address (loop ID) that
determines its priority when it uses the loop to talk.
✓ FC-SW: Fibre Channel-Switched Protocol is used by two devices commu-
nicating on a Fibre Channel switch. Switch ports are connected over a
backplane, which allows any device on the switch to talk to any other
device on the switch at the same time. Many conversations can occur
simultaneously through the switch. A switched fabric is created by con-
necting Fibre Channel switches; such a fabric can have thousands of
devices connected to it.
Each device in a fabric has an address called a World Wide Name (WWN)
that’s hard-coded at the factory onto the host bus adapter (HBA) that
goes into every server and every storage port. The WWN is like the
telephone number of a device in the fabric (or like the MAC address of
a network card) When the device is connected to the fabric, it logs in to
the fabric port, and its WWN registers in the name server so the switch
knows it’s connected to that port. The WWN is also sometimes called a
WWPN, or World Wide Port Name.
The WWN and a WWPN are the exact same thing, the actual address
for a Fibre Channel port. In some cases, large storage arrays can also
have what is known as a WWNN, or World Wide Node Name. Some Fibre
Channel storage manufactures use the WWNN for the entire array, and
then use an offset of the WWN for each port in the array for the WWPN.
I guess this is a Fibre Channel storage manufactures way of making the
World Wide Names they were given by the standards bodies last longer.
You can think of the WWNN as the device itself, and the WWPN as the
actual port within the device, but in the end, it’s all just a WWN.
The name server is like a telephone directory. When one device wants
to talk to another in the fabric, it uses the other device’s phone number
to call it up. The switch protocol acts like the telephone operator. The
first device asks the operator what the other device’s phone number is.
The operator locates the number in the directory (the name server) in
the switch, and then routes the call to the port where the other device is
There is a trick you can use to determine whether the WWN refers to a
server on the fabric or a storage port on the fabric. Most storage ports’
WWN always start with the number 5, and most host bus adapters’ start
with either a 10 or a 21 as the first hexadecimal digits in the WWN. Think
of it like the area code for the phone number. If you see a number like
50:06:03:81:D6:F3:10:32, its probably a port on a storage array. A
number like 10:00:00:01:a9:42:fc:06 will be a servers’ HBA WWN.
✓ SCSI: The SCSI protocol is used by a computer application to talk to its
disk-storage devices. In a SAN, the SCSI protocol is layered on top of
either the FC-AL or FC-SW protocol to enable the application to get to
the disk drives within the storage arrays in a Fibre Channel SAN. This
makes Fibre Channel backward-compatible with all the existing applica-
tions that still use the SCSI protocol to talk to disks inside servers. If the
SCSI protocol was not used, all existing applications would have needed
to be recompiled to use a different method of talking to disk drives.
SCSI works a bit differently in a SAN from the way it does when it talks to
a single disk drive inside a server. SCSI inside a server runs over copper
wires, and data is transmitted in parallel across the wires. In a SAN, the
SCSI protocol is serialized, so each bit of data can be transmitted as a
pulse of light over a fiber-optic cable. If you want to connect older parallel
SCSI-based devices in a SAN, you have to use a data router, which acts as a
bridge between the serial SCSI used in a SAN and the parallel SCSI used in
the device. (See “Data routers,” earlier in this chapter, for the gory details.)
Although iSCSI and Infiniband protocols can also be used in storage networks,
the iSCSI protocol is used over an IP network and then usually bridged into
a Fibre Channel SAN. Infiniband, on the other hand, is used over a dedicated
Infiniband network as a server interconnect, and then bridged into a Fibre
Channel SAN for storage access. But the field is always changing: Infiniband
and iSCSI storage arrays are now becoming available, but they still use either
an IP or IB interface rather than FC.
The addressing scheme used in SAN fabrics is quite different than that in SAN
loops. A fabric can contain thousands of devices rather than the maximum
127 in a loop. Each device in the fabric must have a unique address, just as
every phone number in the world is unique. This is done by assigning every
device in a SAN fabric a World Wide Name (WWN).
What in the world is a World Wide Name?
Each device on the network has a World Wide Name, a 64-bit hexadecimal
number coded into it by its manufacturer. The WWN is often assigned via a
standard block of addresses made available for manufacturers to use. Thus
every device in a SAN fabric has a built-in address assigned by a central
naming authority — in this case, one of the standard-setting organizations
that control SAN standards — the Institute of Electrical and Electronics
Engineers (IEEE, pronounced eye triple-e). The WWN is sometimes referred to
by its IEEE address. A typical WWN in a SAN will look something like this:
On some devices, such as large storage arrays, the storage array itself is
assigned the WWN and the manufacturer then uses the assigned WWN as the
basis for virtual WWNs, which add sequential numbers to identify ports.
The WWN of the storage array is known as the World Wide Node Name or
WWNN. The resulting WWN of the port on the storage array is known as the
World Wide Port Name or WWPN. If the base WWN is (say) 20000000C8328F00
and the storage array has four ports, the array manufacturer could use the
assigned WWN as the base, and then use offsets to create the WWPN for each
port, like this:
20000000C8328F01 for port 1
20000000C8328F02 for port 2
20000000C8328F03 for port 3
20000000C8328F04 for port 4
The manufacturers can use offsets to create World Wide Names as long as
the offsets used do not overlap with any other assigned WWNs from the
block of addresses assigned to the manufacturer.
When it comes to Fibre Channel addressing, the term WWN always refers
to the WWPN of the actual ports, which are like the MAC addresses of an
Ethernet network card. The WWPN (now forever referred to as the WWN for
short) is always used in the name server in the switch to identify devices on
The name server
The name server is a logical service (a specialized program that runs in the
SAN switches) used by the devices connected to the SAN to locate other
devices. The name server in the switched fabric acts like a telephone direc-
tory listing. When a device is plugged into a switch, it logs in to the switch (a
process like logging in to your PC) and registers itself with the name server.
The name server uses its own database to store the WWN information for
every device connected to the fabric, as well as the switch port information
and the associated WWN of each device. When one device wants to talk to
another in the fabric, it looks up that device’s address (its WWN) in the name
server, finds out which port the device is located on, and communication is
then routed between the two devices.
Figure 3-5 shows the name server’s lookup operation in action. The arrows
show how the server on Switch 1 (with address 20000000C8328FE6)
locates the address of the storage device on Switch 2 (at address
50000000B2358D34). After it finds the storage device’s address in the name
server, it knows which switch it’s located on and how to get to the device.
When a network gets big enough to have a few hundred devices connected to a
bunch of switches, the use of a directory listing inside the fabric makes sense.
The switches’ name server information can be used to troubleshoot problems
in a SAN. If your device is connected to a switch but doesn’t get registered in
the name server table, then you know that the problem is somewhere between
the server and the switch; you may have a bad cable. (See Chapter 12 for more
SAN troubleshooting tips.)
Note: this article is from book <Storage Area Networks For Dummies®>.