User`s guide

Chapter 3. Implementing the INRANGE FC/9000 Fibre Channel Director 229
Private loop
Private loop is the most basic form of Fibre Channel. Private loops are generally
built using hubs, and communication is handled by passing messages around
the loop from one device to the next in order to gain control of the loop and to
communicate on it. Because each device is dependent upon the device next to it
for communication, private loops tend to be unstable.
For example, adding, removing, or even rebooting a device causes other devices
on the loop to pause. This pause forces every device to examine what changed
in the environment and to adjust for that change. This Loop Initialization Primitive
Sequence (LIP) can be extremely disruptive when data is being transferred at the
same time that a change of state takes place on an arbitrated loop.
In a controlled, stable, and consistent environment, private loop can perform
adequately for many applications. The main reason for its initial popularity is its
relative low cost. However, when performance, stability and management are
required, a higher performing Fibre Channel communication mode is necessary.
Importantly, a private loop is only known to itself. In some cases it is not possible,
unless some form of translation takes place, for a private device to see or be
seen by others.
Public loop
Public loop implies Fibre Channel arbitrated loop with hubs, switch or switches in
place. Public loop adds a number of key advantages over private:
򐂰 Name server — As described in the Fibre Channel standard, the addition of a
simple name server adds instant control and stability to a loop because each
device registers with the name server on login. The name server then takes
over the need to for devices to pause and pass messages to one another
each a time an event happens on the loop. The switch internally controls the
name server.
򐂰 Management — With a name server and switch management software,
devices on a switch can be queried, examined, and brought online and offline.
Errors can be logged and trapped with messages sent to an administrator
who can then take appropriate action.
򐂰 Zoning — With zoning, a SAN can be made more secure by separating and
segregating data and devices. Zoning can be done by individual switch ports,
using name server entries, or by each device’s unique world wide name
(WWN).
򐂰 Ability to address more than 126 Devices — In a loop only configuration, it is
possible to address a maximum of 126 devices, not including the initiator.
Through connection to a fabric, that range can be increased. This is in
comparison to a private loop's 8-bit address which limit a fabrics size.