Preferred lifetime
This is the length of time during which the address
can be used freely as both a source and a
destination address for traffic exchanges with other devices. This
time span is equal to or less than the valid lifetime also assigned
to the address. If this time expires without the address being refreshed,
the address becomes deprecated and should
be replaced with a new, preferred address. In the deprecated state,
an address can continue to be used as a destination for existing communication
exchanges but is not used for new exchanges or as a source for traffic
sent from the interface. A new, preferred address, and its deprecated
counterpart both appear in the show ipv6 vlan <vid>
output as long as the deprecated
address is within its valid lifetime.