McDATA StorageWorks 2/140 - Director Switch Planning Manual page 108

Planning Considerations for Fibre Channel Topologies
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McDATA Products in a SAN Environment - Planning Manual
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— If you have three fabric elements and set two to Principal and
one to Default, the element with the Principal setting that has
the lowest WWN becomes the principal switch.
— If you have three fabric elements and set two to Default and
one to Never Principal, the element with the Default setting and
the lowest WWN becomes the principal switch.
Note that at least one director or switch in a multiswitch 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
is set to Never Principal and the element that was Principal goes
offline, then all of the other ISLs will segment.
NOTE: It is recommended to configure the switch priority as Default.
In the audit log, note 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
multiswitch 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 used in 24-bit Fibre Channel addresses that uniquely
identify source and destination ports in a fabric.
Each fabric element is configured through the Element 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.