OSPF LSA type 3

Question

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.

Answer 1

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.

Answer 2

• First question answer cisco learningnetwork
• Second question answer rfc2328
• If still with doubts, let us know. Cheers!

Answer 3

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.