HP StorageWorks 64 - SAN Director Switch Planning Manual page 84

Planning Considerations for Fibre Channel Topologies
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Note that at least one director or switch in a multi-switch fabric needs to be set
as Principal or Default. If all the fabric elements are set to Never Principal,
all ISLs will segment. If all but one element are set to Never Principal and
the element that was Principal goes offline, then all of the other ISLs will
segment.
HP recommends configuring the switch priority as Default.
In the audit log, note that the Principal setting maps to a number code of 1,
Default maps to a number code of 254, and Never Principal maps to a
number code of 255. Number codes 2 through 253 are not used.
Fabric WWN assignment — The Fabric Manager application identifies
fabrics using a fabric WWN. The fabric WWN is the same as the WWN of the
fabric's principal switch. If a new principal switch is selected because of a
change to the fabric topology, the fabric WWN changes to the WWN of the
newly selected principal switch.
Domain ID assignment — Each director or switch in a multi-switch fabric is
identified by a unique domain ID that ranges between 1 and 31. A domain ID
of 0 is invalid. Numerical domain IDs specified by a user are converted to
hexadecimal format and are used in 24-bit Fibre Channel addresses that
uniquely identify source and destination ports in a fabric.
Each fabric element is configured through the Product Manager application
with a preferred domain ID. When a director or switch powers on and comes
online, it requests a domain ID from the fabric's principal switch (indicating
its preferred value as part of the request). If the requested domain ID is not
allocated to the fabric, the domain ID is assigned to the requesting director or
switch. If the requested domain ID is already allocated, an unused domain ID
is assigned.
If two operational fabrics join, they determine if any domain ID conflicts exist
between the fabrics. If one or more conflicts exist, the interconnecting ISL
E_Ports segment to prevent the fabrics from joining. To prevent this problem,
HP recommends that all directors and switches be assigned a unique preferred
domain ID. This is particularly important if zoning is implemented through
port number (and by default domain ID) rather that WWN.
SAN High Availability Planning Guide