HP StorageWorks 2/140 FW 08.01.00 McDATA Products in a SAN Environment Plannin - Page 110
Frame delivery order, If a single device has multiple F_Port connections to a director
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Planning Considerations for Fibre Channel Topologies 3 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. 3-26 McDATA Products in a SAN Environment - Planning Manual