McDATA StorageWorks 2/140 - Director Switch Planning Manual page 110

Planning Considerations for Fibre Channel Topologies
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McDATA Products in a SAN Environment - Planning Manual
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Channel frames between devices attached to the fabric and enable
operation of the fabric services firmware on each director or
switch.
Paths are determined when the fabric topology is determined and
remain static as long as the fabric does not change. If the fabric
topology changes (elements are added or removed or ISLs are
added or removed), directors and switches detect the change and
define new data transfer paths as required. The algorithm that
determines data transfer paths is distributive and does not rely on
the principal switch to operate. Each director or switch calculates
its own optimal paths in relation to other fabric elements.
Only minimum-hop data transfer paths route frames between
devices. If an ISL in a minimum-hop path fails, directors and
switches calculate a new least-cost path (which may include more
hops) and route Fibre Channel frames over that new path.
Conversely, if the failed ISL is restored, directors and switches
detect the original minimum-hop path and route Fibre Channel
frames over that path.
When multiple minimum-hop paths (ISLs) between fabric
elements are detected, firmware balances the data transfer load
and assigns ISL as follows:
— The director or switch assigns an equal number of device
entry ports (F_Ports) to each E_Port connected to an ISL. For
example, if a fabric element has two ISLs and six attached
devices, the load from three devices is transferred through
each ISL.
— If a single device has multiple F_Port connections to a director
or switch, the switch assigns the data transfer load across
multiple ISLs to maximize device availability.
• Frame delivery order - When directors or fabric switches
calculate a new least-cost data transfer path through a fabric,
routing tables immediately implement that path. This may result
in Fibre Channel frames being delivered to a destination device
out of order, because frames transmitted over the new (shorter)
path may arrive ahead of previously-transmitted frames that
traverse the old (longer) path. This causes problems because
many Fibre Channel devices cannot receive frames in the
incorrect order.