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.