HP Surestore 64 Planning Guide - Page 94

Path selection, Frame delivery order, connected to an ISL. For example

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• Path selection Directors are not manually configured with data transmission paths to each other. Participating directors automatically exchange information to determine the fabric topology and resulting minimum-hop data transfer paths through the fabric. These paths route Fibre Channel frames between devices attached to the fabric and enable operation of the fabric services firmware on each director. 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 (directors are added or removed or ISLs are added or removed), directors 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 calculates its own optimal paths in relation to other directors. Only minimum-hop data transfer paths route frames between devices. If an ISL in a minimum-hop path fails, directors 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 detect the original minimum-hop path and route Fibre Channel frames over that path. When multiple minimum-hop paths (ISLs) between directors are detected, the switch firmware balances the data transfer load and assigns ISL as follows: - The director assigns an equal number of device entry ports (F_Ports) to each E_Port connected to an ISL. For example, if a director 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, the switch assigns the data transfer load across multiple ISLs to maximize device availability. • Frame delivery order When directors 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. 80 Multiswitch Fabric Support

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80
Multiswitch Fabric Support
•
Path selection
Directors are not manually configured with data transmission paths to each other.
Participating directors automatically exchange information to determine the fabric
topology and resulting minimum-hop data transfer paths through the fabric. These
paths route Fibre Channel frames between devices attached to the fabric and enable
operation of the fabric services firmware on each director.
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 (directors are added or
removed or ISLs are added or removed), directors 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
calculates its own optimal paths in relation to other directors.
Only minimum-hop data transfer paths route frames between devices. If an ISL in a
minimum-hop path fails, directors 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 detect the original minimum-hop path and route Fibre
Channel frames over that path.
When multiple minimum-hop paths (ISLs) between directors are detected, the switch
firmware balances the data transfer load and assigns ISL as follows:
–
The director assigns an equal number of device entry ports (F_Ports) to each E_Port
connected to an ISL. For example, if a director 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, the switch assigns
the data transfer load across multiple ISLs to maximize device availability.
•
Frame delivery order
When directors 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.