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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.

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    You'll find all the answers in RFC 2328 – Ron Trunk Jan 1 '18 at 15:02
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Feb 21 '18 at 16:35
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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. The

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Why LSA type 3 is called summary LSA

As you are already aware, In OSPF routers are divided into areas. Routers within the area know all the information about the fellow router through LSA 1 and 2. This database is fairly detailed as the router can interpret the entire topology of the area using the LSAs. On the contrary the only infomation available to the router for router in other area is:

This router is belongs to Area XX and I can reach this router via ABR router YY.

The router is not aware how the topology is in that area or if the ABR is directly connected to the other router. This is because when an ABR sends LSA type 3, It only sends the necessary information necessary to enable communication. In a way it summarises the routes. Hence the name summary LSA

**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?*

The information about all the routers have to be there, otherwise how can we ensure connectivity between all users in the network. As to what exactly does LSA 3 contain. You can check a wireshark packet at packetlife.com or create a network yourself on GNS3. But for understanding purposes, what the ABR packet says is: I can reach this network or this address and the cost will be XX. But it does not tell how will it reach the network. Or where is the network.

when does the ABR send LSA type 3

A router will be an ABR when it is connected to area 0 and some other area. So it will have OSPF running on multiple interfaces one of them will be in area 0. Other interfaces can belong to any area.

Now consider only area 0. In area 0 the ABR will send LSA 1 and 2 like all other routers to have the complete information of the topology.

Let the other area be area 1, On the interface connected to area 1, the ABR will sned and recieve LSA 1 and 2 to get the entire view of area 1. Now the summarise area 0 and send 1 LSA type 3 to area 1

Similarly it will summarise area 1 and send one LSA type 3 in area 0.

For a better understanding see this video on OSPF areas here

To clarify stuff, by having a topology view I mean a router will know where an interface of a given router is connected to.

  • I get it now.. But there's one small thing. Let's say there are 2 routers(r0 and r1) in area 1 and 2 routers(r2 and r3) in area 0 and r4 as an ABR. Now, suppose a network or a link in r0 goes down, then r0 updates the same to r1 and r4 using LSA type 1. So when r4 receives it and makes a change to the database, does it send an LSA type 3 to the area 0? Because, as you have rightly said, routers in 1 area do not need to know the complete topology or link details of routers present in the other area. – RRHS Feb 4 '18 at 9:48
  • So to make my question short, does an ABR send a LSA type 3 to other area whenever there is a change in that area or does it send an LSA type 3 once it receives all the info about the Routers present in that area.. What I want to know is, how frequently or on what basis does an LSA type 3 gets sent by the ABR from one area to the another. – RRHS Feb 4 '18 at 9:49
  • To answer your question, OSPF is LSA protocol. Whre LSA is "link state" algorithm. So what OSPF actually conveys is the status and connectivity of the interfaces, the routes are created by the router itself .. locally.(hence called a distributed protocol). So YES it will send a the update. Because there is a change in the link status and OSPF routers need to update it] – john Feb 5 '18 at 21:32

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