There is a misunderstanding when it comes to ospf flooding mechanism. This concept is not only applicable to ospf, it is used by all link state protocol like IS-IS. To better understand ospf, we should first get a good understanding of the flooding mechanism and the flooding scope of different type of ospf LSA.
Flooding means take from one interface and send out to others interfaces. Sounds strange because we all know that this is somehow the behavior of the broadcasting when we talk about the layer two network.
In ospf, we do have the same type of behavior when it comes to lsa flooding. We must pay carefully attention that without the flooding mechanism in the ospf process, we would not have databases synchronized in the entire area, especially when our area is running multiple types of ospf network.
This link helps on understanding the process: http://www.ciscopress.com/articles/article.asp?p=24090&seqNum=4
Information exchange between ospf neighbors are contain within ospf packet, (Hello. DBD, LSR, LSU, LSAk).
In a shared segment (with ospf network type broadcast), if a router has a link failure, it will notice this state to the DR using the multicast address 224.0.0.6. This information is communicate via a LSU which contains the “Type one LSA” describing each link of the router. The DR, upon received the LSU, is going to unpack it, process it and send a new LSU which now contain the same “LSA type 1″ previously learned and a new ” lsa Type 2 describing the relationship between ospf routers on the segment. This new Type 2 Lsu is sent on the segment using the multicast address 224.0.0.5 Because this address is a link local address, every packet sent to this address ends up into the receiving router and cannot go further, but thank to flooding mechanism and thank to the fact that databases in the area must be the same, information contained inside the LSA can be flooded if the receiving router is sitting between two segments in the same area. If this is the case, the router will process the lsu by sending it out others interfaces. Having said that, the origination LSA from one segment just crossed the router: that is the flooding.
It is very important to understand that, especially when it comes to designing, where some questions need to be addressed first:
How many area do we need?
How important is the numbers of the adjacencies within the area?
Do we need to create multiple segments and keep a small size of packets or it is better to have only one DR in a single area which might cause some problems relate to ospf MTU.
I hope this helps.
Blaise Annesti NGADJUI
Network Engineering : “I hear and I forget. I see and I remember. I do and I understand.” ~Confucius 