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Preferred path - Preferred path is an option that allows a user to
configure an ISL data path between multiple fabric elements
(directors and fabric switches) by configuring the source and exit
ports of the origination fabric element and the Domain_ID of the
destination fabric element. Each participating director or switch
must be configured as part of a desired path. For information
about the feature, refer to
ATTENTION ! Activating a preferred path can result in receipt of out-of-
order frames if the preferred path differs from the current path, if input and
output (I/O) is active from the source port, and if congestion is present on the
current path.
In general, Fibre Channel frames are routed through fabric paths
that implement the minimum possible hop count. For example, in
Figure
3-11, all traffic between devices connected to director S
and director S
communicate directly through ISLs that connect
2
the directors (one hop). No traffic is routed through director S
(two hops). If heavy traffic between the devices is expected,
multiple ISL connections should be configured to create multiple
minimum-hop paths. With multiple paths, the directors balance
the load by assigning traffic from different ports to different
minimum-hop paths (ISLs).
When balancing a load across multiple ISLs, a director or switch
attempts to avoid assigning multiple ports attached to a device to
the same ISL. This minimizes the probability that failure of a
single ISL will affect all paths to the device. However, because
port assignments are made incrementally as devices log into the
fabric and ISLs become available, optimal results are not
guaranteed.
Special consideration must also be given to applications with
high data transfer rates or devices that participate in frequent or
critical data transfer operations. For example, in
suppose device D
is a server and device D
7
both devices participate in a critical nightly backup operation. It
is recommended that such a connection be routed directly
through director S
2
port connections, WWN-bound port connections, or a preferred
path. For additional information, refer to
page 3-34.
Planning Considerations for Fibre Channel Topologies
Planning Considerations for Fibre Channel Topologies
Preferred
Path.
(rather than the entire fabric) through zoned
Device Locality
1
3
Figure
3-11,
is a storage unit and
9
on
3
3-23
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