Router – OSPF LSA type 3

ospfroute-summarizationrouter

This is what I know about LSA types from what I've learnt.

  1. LSA type 1 is circulated inside an area and doesn't get to other areas.

  2. LSA type 3 is sent from one area(From backbone to non-backbone and vice-versa) to the other area by the ABR.

But the following are my questions

  • Why is LSA type 3 is called summary LSA if the routers in one area know about the routers present in the other area?

  • From what I've learnt, ABR summarises the area but when I get into the database of a router in a particular area, I see
    there is info about all the routers present in another area rather than the network summarisation. So what does LSA type 3 actually contain sent by the ABR?

  • And one final question, when does the ABR send LSA type 3? Does it send for each LSA type 1 received from a router. For ex: If there are 5 routers in an area, does the ABR send 5 LSA type 3 to the other area for every LSA type 1 it receives from each of the 5 routers.
    Or does it send LSA type 3 after it reaches to a conclusion that it has received all the 5 LSA type 1 from the routers present in the network and then send the LSA type 3 to the other area only once.

Best Answer

LSA 3 summarizes the TOPOLOGY, not the routes.

If you look at the ospf database within an area 0 router, there should be a type 3 LSA for each ROUTE/prefix within the other area(s). There is a key distinction between this vs knowing about the ROUTERS in a Type 1 LSA. 

Type 1 LSA = Router (NODE) in the OSPF GRAPH + links

Type 3 LSA = ROUTE from other area advertised by ABR

Thus, Area 0 does NOT know about the ROUTERS in the other areas, only the prefixes/ROUTES that exist within that area as advertised by the ABR. Likewise, the other areas will only know about the ROUTES in Area 0/additional areas, not the ROUTERS.

The type 3 LSA will include the advertising router (ABR) which is used to forward traffic toward. This is why Inter-Area routing in OSPF is more like Distance Vector. The traffic to other areas must flow through the ABR as the next-hop and don't know the topology past that point in the network.

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