The Fibre Channel domain (fcdomain) feature performs principal switch selection, domain ID distribution, FC ID allocation, and fabric reconfiguration functions as described in the FC-SW-2 standards. The domains are configured on a per VSAN basis. If you do not configure a domain ID, the local switch uses a random ID.
This section describes each fcdomain phase:
*

Principal switch selection - This phase guarantees the selection of a unique principal switch across the fabric.

* Domain ID distribution - This phase guarantees each switch in the fabric obtains a unique

domain ID.
*

FC ID allocation - This phase guarantees a unique FC ID assignment to each device attached to the corresponding switch in the fabric.
* Fabric reconfiguration - This phase guarantees a resynchronization of all switches in the fabric to ensure they simultaneously restart a new principal switch selection phase.
Reference:
http://www.cisco.com/c/en/us/td/docs/switches/datacenter/mds9000/sw/5_2/configuration/g uides/sysmgnt/DCNM-SAN/sysmgmt_dcnm/sysmgmt_overview.html#wp1051962