HP StorageWorks 64 FW 07.00.00/HAFM SW 08.06.00 McDATA Products in a SAN Envir - Page 113
Frame delivery order, many Fibre Channel devices cannot receive frames in
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Planning Considerations for Fibre Channel Topologies 3 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-oforder 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 3-27