All LSDB - Link State Database in all routers participating in OSPF domain need to be exactly the same. That is main assumption of SPF Dijkstra's Algorithm and using that argument SPF is able to calculate Shortest Path Tree to all destinations in domain.
That is why the most important role depend from correct LSA flooding in domain.
Each individal LSA generated by any routers need to be acknowledge. This may be accomplished by either an :
1) Implicit ack
2) Explicit ack
- All duplicate LSA is received from a neighbor possibly indicating that it has not yet received an ack. That is why router put that LSA into Link State Retransmission list of every neighbor to which it was sent. If router doesn't receive ack it will retransmis his LSA every Rxmt Internal.
- LSA contains 3 values : seq number, checksum and age. All of them are responsible for keeping all LSDB (Link State Database) in whole OSPF domain updated and exactly the same.
- OSPF uses 32-bit signed, linear seq number from Initial Sequence Number to Max Sequence Number. When router send his first LSA it sets the value for Initial Sequence Number and increments the seq number by one.
Only the router that is originated the LSA can permanently age it or modify it!
There is also mechanism which is preventing legitimate from reaching MaxAge. In brief if nothing is happening on the network, there are no faults we don't have flush whole OSPF LSDB every 1 hour. That is why Link State Refresh is set up for 30 mins. Each half an hour known as LSRefresh Time the router that originate LSA flood a new copy of the LSA with and increment seq number and age of zero.
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