an OSPF LSA is a data structure that contains routing information. There are several types of LSAs: router LSA, network LSA, area summary LSA, ASBR summary LSA, external LSA type 1, external LSA type2.
During OSPF loading phase the pair of routers that are trying to synchronize their database exchange information about the LSA headers:
each LSA is identified by:
the OSPF router-id of advertising router (an ip address)
the LSA sequence number ( a 32 bit number starting from a non zero value each LSA has its own sequence number)
an age in seconds that say how much old is this LSA.
the two parts take note of missing or newer information from the counterpart.
At the end one router makes a series of link state request : LSR
give all the details of LSA with sequence number S, advertsing router R
the other part answers using an OSPF link state update:
the link state update is the only packet that contains the full details of the requested LSA.
all this messages use explicit acknowledges to signal correct reception of requested information.
so LSA is a data structure that can be transported in full detail in LSU packets.
other OSPF messages carry the so called LSA header made of the three fields described above.
to be noted: an OSPF LSU packet can carry multiple LSA bodies for link efficiency up to interface MTU (including IP and OSPF headers)
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