This field is the checksum of the complete contents of the advertisement, excepting the LS age field. The LS 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. It is documented in Annex B of [RFC 905]. The link state advertisement header also contains the length of the advertisement in bytes; subtracting the size of the LS age field (two bytes) yields the amount of data to checksum.
The checksum is used to detect data corruption of an advertisement. This corruption can occur while an advertisement is being flooded, or while it is being held in a router's memory. The LS checksum field cannot take on the value of zero; the occurrence of such a value should be considered a checksum failure. In other words, calculation of the checksum is not optional.
The checksum of a link state advertisement is verified in two cases: a) when it is received in a Link State Update Packet and b) at times during the aging of the link state database. The detection of a checksum failure leads to separate actions in each case. See Sections 13 and 14 for more details.
Whenever the LS sequence number field indicates that two instances of an advertisement are the same, the LS checksum field is examined. If there is a difference, the instance with the larger LS checksum is considered to be most recent.[12] See Section 13.1 for more details.