HP 316095-B21 - StorageWorks Edge Switch 2/24 Planning Manual page 87

SAN High Availability Planning Guide
Path selection — Directors and switches are not manually configured with
data transmission paths to each other. Participating fabric elements
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 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 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.
Planning Considerations for Fibre Channel Topologies
87