Thursday, July 26, 2007

FC- Addressing

FC- Addressing

All devices in a Fibre Channel environment have an identity. The way that the
identity is assigned and used depends on the format of the Fibre Channel fabric.
98 Introduction to Storage Area Networks
For example, there is a difference between the way that addressing is done in an
arbitrated loop and a fabric.

1 World Wide Name

All Fibre Channel devices have a unique identity called the World Wide Name
(WWN). This is similar to the way all Ethernet cards have a unique MAC address.
Each N_Port will have its own WWN, but it also possible for a device with more
than one Fibre Channel adapter to have its own WWN as well. Thus, for
example, an IBM TotalStorage Enterprise Storage Server has its own WWN as
well as incorporating the WWNs of the adapter within it. This means that a soft
zone can be created using the entire array, or individual zones could be created
using particular adapters. In the future, this will be the case of the servers as
This WWN is a 64-bit address, and if two WWN addresses are put into the frame
header, this leaves 16 bytes of data just for identifying destination and source
address. So 64-bit addresses can impact routing performance.

2 WWN and WWPN

Each device in the SAN is identified by a unique world wide name (WWN). The
WWN contains a vendor identifier field, which is defined and maintained by the
IEEE, and a vendor-specific information field.
Currently, there are two formats of the WWN as defined by the IEEE. The original
format contains either a hex 10 or hex 20 in the first two bytes of the address.
This is then followed by the vendor-specific information.
The new addressing scheme starts with a hex 5 or 6 in the first half-byte followed
by the vendor identifier in the next 3 bytes. The vendor-specific information is
then contained in the following fields.
Some devices may have multiple Fibre Channel adapters, like an ESS, for
example. In this case the device also has an identifier for each of its Fibre
Channel adapters. This identifier is called the world wide port name (WWPN). It
is possible to uniquely identify all Fibre Channel adapters and paths within a

3 Port address

Because of the potential impact on routing performance by using 64-bit
addressing, there is another addressing scheme used in Fibre Channel
networks. This scheme is used to address ports in the switched fabric. Each port
Chapter 3. SAN fabrics and connectivity 99
in the switched fabric has its own unique 24-bit address. With this 24-bit address
scheme, we get a smaller frame header, and this can speed up the routing
process. With this frame header and routing logic, the Fibre Channel is optimized
for high-speed switching of frames.
With a 24-bit addressing scheme, this allows for up to 16 million addresses,
which is an address space larger than any practical SAN design in existence in
today’s world. There needs to be some relationship between this 24-bit address
and the 64-bit address associated with World Wide Names. We will explain this
in the section that follows.

4 24-bit port address

The 24-bit address scheme removes the overhead of manual administration of
addresses by allowing topology itself to assign addresses. This is not like WWN
addressing, in which the addresses are assigned to the manufacturers by the
IEEE standards committee, and are built in to the device at the time of
manufacture. If the topology itself assigns the 24-bit addresses, then somebody
has to be responsible for the addressing scheme from WWN addressing to port
In the switched fabric environment, the switch itself is responsible for assigning
and maintaining the port addresses. When the device with its WWN logs into the
switch on a specific port, the switch will assign the port address to that port and
the switch will also maintain the correlation between the port address and the
WWN address of the device of that port. This function of the switch is
implemented by using the Name Server.
The Name Server is a component of the fabric operating system, which runs
inside the switch. It is essentially a database of objects in which fabric-attached
device registers its values.
Dynamic addressing also removes the partial element of human error in
addressing maintenance, and provides more flexibility in additions, moves, and
changes in the SAN.

5 Loop address

An NL_Port, like an N_Port, has a 24-bit port address. If no switch connection
exists, the two upper bytes of this port address are zeroes (x’00 00’) and referred
to as a private loop. The devices on the loop have no connection with the outside
world. If the loop is attached to a fabric and an NL_Port supports a fabric login,
the upper two bytes are assigned a positive value by the switch. We call this
mode a public loop.
100 Introduction to Storage Area Networks
As fabric-capable NL_Ports are members of both a local loop and the greater
fabric community, a 24-bit address is needed as an identifier in the network. In
this case of public loop assignment, the value of the upper two bytes represents
the loop identifier, and this will be common to all NL_Ports on the same loop that
performed login to the fabric.
In both public and private Arbitrated Loops, the last byte of the 24-bit port
address refers to the Arbitrated Loop physical address (AL_PA). The AL_PA is
acquired during initialization of the loop and may, in the case of a fabric-capable
loop device, be modified by the switch during login.
The total number of the AL_PAs available for Arbitrated Loop addressing is 127.
This number is based on the requirements of 8b/10b running disparity between