EMC Connectrix EC-1200 Planning Manual page 52

Fibre Channel Switching Concepts
2
To change a Domain ID, a switch must be taken off line. Some HBAs
(JNI on Solaris and Emulex on Windows NT, for example) can handle
a Domain ID change without requiring device driver adminstration
changes. In some operating environments, however (currently
HP-UX and AIX), drivers require device driver-level administrative
changes when the Domain ID is changed.
If two operational fabrics join, they determine if any Domain ID
conflicts exist between the fabrics. If a conflict exists, the interswitch
link (ISL) E_Ports segment themselves from each other, preventing
the fabrics from joining. To avoid this problem, assign a unique
Preferred Domain ID to each switch in a data center. If there are plans
to go data center to data center, apply this uniqueness across both
data centers. This will facilitate future fabric mergers, and is
particularly important if zoning is implemented through port
number (and by default Domain ID) rather than WWN.
EMC recommends zoning by WWN. This allows moving fiber-optic
cables from one port to another without invalidating the zoning
configuration.
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Connectrix v6.02 Planning Guide