Hello,
You are absolutely not bothering - don't worry. These forums are here to talk about things over and over until they can be understood.
I cant understand what happend every 30 minutes among each routers .
Okay, let's try to approach this from a different angle, and I will start in a very general way.
A key term in OSPF is the notion of a Link State Advertisement, or LSA. An LSA is used to carry information about elements of your network topology - routers and their connections to their neighboring objects including the network addresses, multiaccess networks, prefixes from other areas, external prefixes from outside of our OSPF domain. Information stored in these LSAs is used by each router to construct a "map", or a topological map (a directed graph, as mathematicians use to call it) of your network. This map is then used to search for the shortest paths.
LSAs are flooded to each router within a particular flooding scope - some LSAs may be flooded to routers in their own area only, other LSAs can be flooded even across areas, possibly to all routers within the OSPF domain. Each router generates one or more LSAs, depending on type and amount of the information it needs to tell to the other routers. Other routers will store these LSAs and flood them further but they may not modify them. Each router may modify only the LSAs it has originated, and if it modifies them, they need to be reflooded. However, a router is absolutely prohibited from modifying other routers' LSAs.
Because OSPF is an event-driven protocol, it would flood new information only when a topology change occured. If the topology did not change, OSPF would not flood any LSAs after the initial synchronization. This may seem like a straightforward way to implement OSPF but there is an important gotcha: what if the lack of new LSAs is caused by the fact that their originating router is dead?
Clearly, it is not a good idea to allow LSAs to stay in routers' link-state databases forever. For whatever reason, their originator may become unable to announce new LSAs, or it may become disconnected. However, LSAs it has originated would be lurking in link-state databases indefinitely.
That is why each and every LSA has an age timer associated with it. This timer is set to 0 when a router originates an LSA and floods it. Each other router then stores this foreign LSA in its link-state database and increments its age each second. If the age reaches 3600 seconds, or 1 hour, the router will flush this foreign LSA from its link-state database and recompute the routing table, bypassing the object that was described by the flushed LSA. In simple terms, each router must refresh its own LSAs before they reach the maximum age of 3600 seconds, otherwise other routers will flush them. A router can refresh a self-originated LSA by flooding it anew with the age timer set to 0 and the sequence number of the LSA being incremented. Do not confuse the LSA sequence numbers and their age. The sequence number of an LSA is just a revision number - it starts at a certain value and each new version (or better said, generation) of the same LSA carries an incremented sequence number. The sequence numbers are used to determine which LSA is more recent and more up-to-date when flooding updated LSAs. The age expresses the time since the LSA of this particular revision was last updated, and is used to limit the maximum time this LSA can stay in a link-state database.
So in short, before this 3600 second maximum age timer expires, a router must refresh its self-originated LSAs by flooding them anew, incrementing their sequence number by 1 and setting the age timer to 0. Obviously, it is too late to do it on or after the 3600 second period - by that time, other routers will have flushed the LSA already. In order to refresh LSAs smoothly, you need to refresh them well before the 3600 second period expires. OSPF takes a very conservative approach here and performs the refresh every 1800 seconds, or every 30 minutes.
This is the 30 minute interval which you are asking about. As the age of some self-originated LSA approaches 1800 seconds, the router will generate a refreshed version of the same LSA and flood it - an incremented sequence number, the age timer set to 0. That's it. Nothing more. There is no some kind of resynchronization of entire link-state databases or something. You just flood a new LSA, just like you would flood an LSA if some topological event occured. The DBD packets have nothing to do here. As with all LSA flooding operations, LSAs are flooded within LSU packets, and they are confirmed with LSAck packets.
You have also asked what will happen if the 1800 second period expires. Nothing will happen because the LSAs are going to be valid for another 1800 seconds (remember, their max age is 3600 seconds - that is an OSPF constant). You could refresh your LSAs every 300 or 3000 seconds if you wanted to. Even refreshing them every 3590 seconds would work but it diminishes your reserve for dealing with unexpected delays in processing the LSAs in a graceful way.
From here ,Do you mean
Routers just flood their all LSAs every 30 minutes to reset the age ?
Yes That is what I wanted to say, and you probably now know why I was saying that
Of course please feel welcome to ask further.
Best regards,
Peter
