According to Cisco, OSPF area is a collection of logical OSPF networks (routers and links) that have the same area identification. Depending upon the design of your network, your routers can be part of a single area or multiple areas. Why do we have to complex things and put the router in different areas? Actually, there are many reasons, and one of the many reasons is that it reduces the size of OSPF database when OSPF network is divided into different areas. Think of it this way, dividing the routers into different areas, reduces the size of the database, reduces the frequency of SPF calculation and smaller routing table. Thus, fewer requirements on router memory and CPU. I am not saying that SPF calculation is exhausting the CPU and router memory but the sending and flooding of the new topology information does.

OSPF AREA TYPES

No matter what your OSPF design in your network, all areas you have created should be connected to the backbone area. Backbone area or Area 0 (0.0.0.0) is like the meeting points of all these non-backbone areas. So, it is not just an option or recommendation but a must in every OSPF network design. Although there is an option called virtual link, it is not a good network design but just a remedy to a broken OSPF design.

Now, not all LSA Types are allowed in different areas. Some LSA Types are restricted and depend upon which area type is configured. This further reduces the link state database and routing tables. Injecting too many external routes are also memory intensive. However, all LSA types are allowed in the Normal area. An area which is not configured as Stub, Totally Stub, NSSA, NSSA Totally Stub area is called Standard/Normal Area. We can say that the backbone area is behaving like a Normal /Standard Area as LSA Type 1-5 are not being restricted in this area. When the network is divided into different area numbers (non-zero), an ABR is used to connect the Standard area to the Backbone Area.

On my next post, I will introduce some of the area types which have their own rules in restricting LSA Types.

LSA Type 1 or called Router LSA, as the name implies, represents the router. Each router within an OSPF area will flood LSA Type 1 and it stays within an area.

Using the “show ip ospf database” command, it shows the router ID of directly connected neighbor and other information about its neighbors of adjacent routers within the same area. Multiple areas can be seen in ABR OSPF database.

LSA Type 2 or Network LSA is the LSA generated by designated router (DR). It contains information about the DR and lists all routers it is adjacent to. If the Link ID in LSA Type 1 is the router ID, the Link ID in LSA Type 2 is the interface IP address of the designated router. And, like the LSA Type 1, it is flooded between neighbors in the same area. This type of LSA can be seen on NBMA and Broadcast networks where DR/BDR is elected.

LSA Type 3 is the ABR Summary LSA. This LSA is generated by the Area Border Routers (ABR) to advertise network from one area to another area. It contains information about inter-area routes. If you see O IA entries in the routing table, these are Type 3 LSAs.

Looking at the screenshot above, the O IA are Type 3 LSAs which are advertised by ABR. Aedes is connected to my Anopheles ABR which has a router ID of 192.168.2.1. Take a look at the screenshot below to further check if it is really the Anopheles ABR advertising the Type 3 LSA route 192.168.12.0.

You can check the advertising routers for all OSPF LSA types by utilizing the “show ip ospf database.” You can further add more parameters whether you want to check advertising router of the external, network, summary, router, and other LSA types.

LSA Type 4 is the ASBR Summary LSA. This LSA is generated by the ABR that contains routes to Autonomous System Border Routers (ASBR). It identifies the ASBR IP address and location. Once ABR received information about ASBR, it will advertise LSA Type 4 and inject the route to area 0. Please note that ASBR itself does not generate LSA Type 4 rather it will generate LSA Type 1 regarding its networks.

LSA Type 5 is the External LSA. This LSA is generated by ASBR. ASBR creates a type 5 LSA for a subnet that is injected into OSPF from an external source or via a redistribute command. As it propagates to the network, the advertising router ID, which is the ASBR router ID, is not changed so router uses Type 4 LSA to know how to reach that ASBR. In the routing table, Type 5 LSA are the O E1 and O E2 entries.

LSA Type 6 is called Multicast LSA which is not generally in used today since the introduction of OSPFv3. To tell you honestly, I don’t even know how it was used before.

LSA Type 7 is the Not-So-Stubby-Area (NSSA) External LSA. This LSA is generated by NSSA ASBR. In an NSSA area, external routes are carried as Type 7 LSA. LSA Type 5 is somehow same with LSA Type 7 as they carry external routes information. However, LSA Type 5 is not allowed on stub areas that is why LSA Type 7 is used as a ploy. LSA Type 7 stays within an NSSA area in order for other routers in other areas know about the external routes, the NSSA ABR will convert the Type 7 LSA to Type 5 LSA and flooded in the area 0 and to the other areas in OSPF domain. If there are one or two NSSA ABRs, the NSSA ABR who has the highest router ID will be the one to do the translation.

LSA Type 8 is the Link Local LSA for OSPFv3 while LSA Type 9 – 11 are Opaque LSAs. These are OSPF LSA extensions used for MPLS traffic engineering. I haven’t tried to lab these LSA types but I will somehow in the future.