LSA 1 send only to the DR or to all Routers in the OSPF Area?

FrankGuthrie

So, I'm still confused about LSA 1 and LSA 2.



Are the LSA 1's flooded to all Router in the same OSPF Area, or are they just send to the DR? It seem logical that LSA 1's are not flooded but send to the DR, and from there the DR makes 1 types 2 LSA and floods these to all the router in the Area. Like the pictures below:

Or is it like this (LSA1 sends between all Routes in the same OSPF area):



For me the second scenario doesn't make sense as this means the Router don't need the LSA type 2 to built the graph...

fredrikjj

Your main problem is probably that you think of flooding as somehow connected to the LSAs. Every router should have an identical link state database that describes the topology. Each LSA describes a small part of the topology, but when you have all of them, you get a complete map. The flooding procedures make sure that the identical databases are achieved. The flooding itself doesn't change the LSAs, and it's something that happens behind the scenes.

If we look at for example Routing TCP/IP Volume 1, page 363, there's a few pages on how this actually work. In short:

• LSAs are put in a packet called Link State Update. Each packet can carry multiple LSAs.
• A Link State Update is sent to a neighbor using multicast or unicast, depending on some factors.
• Neighbor then builds a new LSU packet and sends it to its neighbors, etc, but not back out the same interface as it was received.
• On a broadcast network, LSU packets are sent to the DR, which then sends it to the non-DRs. This reduces the flooding compare to if all routers sent the packets to all other routers.
• There's an acknowledgement for the LSU packets, and this uses the Link State Acknowledgement packet.
• There are various nuances to the acknowledgement process that's probably only of interest to protocol developers.

Note that in no part of this process are the LSAs themselves, and what type they are, mentioned.