HP StorageWorks 64 - SAN Director Switch Planning Manual page 85

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SAN High Availability Planning Guide
When assigning preferred domain IDs in an open fabric with directors and
switches supplied by multiple OEMs, be aware of the following:
— For directors and switches, the firmware adds a base offset of 96
(hexadecimal 60) to the numerically assigned preferred domain ID.
Therefore, if a user assigns a director or switch a numerical preferred
domain ID of 1, the firmware assigns a hexadecimal domain ID of 61.
— For non-HP directors and switches, the product firmware may not add a
base offset to the numerical preferred domain ID, or may add a different
hexadecimal base offset (not 20 or 60).
As a consequence of this variable base offset and hexadecimal conversion,
domain ID conflicts may exist in an open fabric, even if each participating
director and switch is assigned a unique numerical domain ID. To determine
the method of preferred domain ID assignment for a product, refer to the
supporting OEM publications for the product, or contact HP.
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.
Planning Considerations for Fibre Channel Topologies
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