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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.
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.
A rerouting delay parameter can be enabled at the Element
Manager application to ensure a director or switch provides
correct frame order delivery. The delay period is equal to the error
detect time out value (E_D_TOV) specified in the Element
Manager application. Class 2 frames transmitted into the fabric
during this delay period are rejected; Class 3 frames are discarded
without notification. By default, the rerouting delay parameter is
disabled.
NOTE: To prevent E_Port segmentation, the same E_D_TOV and
resource allocation time out value (R_A_TOV) must be specified for each
fabric element.
Planning Considerations for Fibre Channel Topologies
Planning Considerations for Fibre Channel Topologies
3
3-27
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