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I'm confused about how OSPF routers perform LSA to create LSDB.

I suppose, an OSPF router will exchange LSA only with the routers to which it has established adjacencies right?

What adjacencies do is permit direct exchange of route updates between OSPF routers. Generally all routers on shared network will establish adjacencies with DR and BDR which makes sure that LSDB are synchronized.

So, in order to create LSDB, LSA has to be flooded in a network.

How is it possible? Will LSA be exchanged only to DR and BDR?

Can somebody explain it to me in detail?

  • Please let me know if you still have any concerns regarding my answer to your question? – Hung Tran Sep 25 '17 at 13:30
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How is it possible? Will LSA be exchanged only to DR and BDR?

DR and BDR election happens in Multiaccess Networks/Segments. And other routers establish full adjacencies with these DR and BDR only.

In this situation, LSA is NOT flooded in the network by non-DR routers but DR.

When there is a change in a link state, a non-DR router multicasts update packets (LSU) to DR and BDR (at multicast address - 224.0.0.6). DR receives and floods the update packets (LSU) to its adjacent routers (at multicast address - 224.0.0.), thus the LSDB is synchronized.

I hope it is helpful and answer your question.

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How an OSPF router forms an adjacency depends on the network type. DRs and BDRs are used in multiaccess (broadcast and non-broadcast) networks. Point-to-point and point-to-multipoint networks do not use DRs and BDRs, but the OSPF routers will form full adjacencies with each neighboring router.

When a network uses DRs and BDRs, an OSPF router will only form full adjacencies with those two routers, but those two routers will exchange with all the OSPF routers on the multiaccess network, so all the OSPF routers on the multiaccess network will have the same information.

A detailed explanation of how this happens is too large for this site, but it is explained in detail in RFC 2328, OSPF Version 2.

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