'This field is the checksum of the complete contents of
the advertisement, excepting the age field. The age
field is excepted so that an advertisement's age can be
incremented without updating the checksum. The
checksum used is the same that is used for ISO
connectionless datagrams; it is commonly referred to as
the Fletcher checksum.'
REFERENCE
'OSPF Version 2, Section 12.1.7 LS checksum'