EMC Connectrix EC-1200 Planning Manual page 54

Fibre Channel Switching Concepts
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If all but one director/switch in the fabric is set to Never Principal,
and the principal switch goes off line, all remaining ISLs will
segment, with reason code 05.
If the principal switch was selected by WWN, adding a switch with a
lower WWN does not change the principal if the newly added switch
is set to Open Fabric 1.0 Interop Mode. This is because adding such a
switch does not cause a Build Fabric process. The next time a Build
Fabric occurs, however, the switch with the lowest WWN will
become principal.
In some logs, Switch Priority settings map to numbers: Principal = 1,
Default = 254, Never Principal = 255.
When building a fabric of a two-tier architecture (backbones and
edges devices, described in more detail under Two-Tier Architecture on
page 4-23), EMC recommends configuring one of the Symmetrix tier
backbones as the principal switch. This strategy has simplicity,
reliability, and efficiency benefits.
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Connectrix v6.02 Planning Guide