HP StorageWorks 64 - SAN Director Switch Planning Manual page 86

Planning Considerations for Fibre Channel Topologies
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Note:
time out value (R_A_TOV) must be specified for each fabric element.
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When multiple minimum-hop paths (ISLs) between fabric elements are
detected, firmware balances the data transfer load and assigns ISLs 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 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 can cause problems because many Fibre Channel
devices cannot receive frames in the incorrect order.
A rerouting delay parameter can be enabled at the Product Manager
application to ensure the 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 Product 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 enabled.
To prevent E_Port segmentation, the same E_D_TOV and resource allocation
E_Port segmentation — When an ISL activates, the two fabric elements
exchange operating parameters to determine if they are compatible and can
join to form a single fabric. If the elements are incompatible, the connecting
E_Port at each director or switch segments to prevent the creation of a single
fabric. A segmented link transmits only Class F traffic; the link does not
transmit Class 2 or Class 3 traffic.
The following conditions cause E_Ports to segment:
— Incompatible operating parameters — Either the R_A_TOV or
E_D_TOV is inconsistent between the two fabric elements.
— Duplicate domain IDs — One or more domain ID conflicts are detected.
SAN High Availability Planning Guide