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4.08.2014

OSPF Areas (Part 2) - Backbone and Normal Areas

Links:

RFC 2328 – OSPF Version 2
RFC 2740 – OSPF for IPv6

Backbone area (Area 0)

Let’s start with the Backbone area. The backbone area is just as the name implies the backbone of the OSPF domain. Think of the backbone area as the hub for OSPF. The backbones main responsibility is to distribute routing information (LSAs) between non-backbone areas.


Breakdown of Backbone Area:

  • Backbone = area  0 (or area 0.0.0.0)
  • Must be contiguous and not split up
  • All area ABRs must connect  to the backbone 
  • Virtual links are used when areas are not physically connected
  • Cannot be configured as stub or NSSA area type
  • Redistribution allowed into the backbone area
  • All LSA types are allowed and propagated


Normal Area:

Normal areas are basically backbone areas. There is really nothing special about them and most resources just glance over them. 

Breakdown of normal Area:

  • All LSA types are allowed and propagated
  • All area ABRs must connect  to the backbone
  • Virtual links are used when areas are not physically connected
  • Redistribution allowed into the backbone area
  • All LSA types are allowed and propagated

Labbing it up


Once again here is the lab setup we are using for this post.  Lets dig in and see what’s going on in the backbone area. 


From the diagram, we can see that R1, R2, and R3 are all backbone routers in area 0. All three router are also ABRs with R1 being an ABR for area 2, and R2 and R3 being ABRs for area 3. Both area 2 and 3 are setup and normal areas.

Using the command show ip ospf you can see the configured area type.


R1 shows area 0 as “BACKBONE (0)" as expected.  You can also see Area 2 does not metion an area type. This tells you the area is a normal area or more specifically it dosen't list any stub or NSSA flags. More on this in the Stub and NSSA posts.

We can also double check this on R2 or R3 for area 3.


Show ipv6 ospf can be used to verify the OSPFv3 area types the exact same way.


Show ip ospf border-routers will show you the ABRs and any ASBRs in the routers configured areas. Notice R2 sees R3 (3.3.3.3) as an ABR for both area 0 and 3 since they reside in both areas.



Show ipv6 ospf border-routers gives the same output. Notice how link-local addresses are being used?



Now for LSAs.

Show ip ospf database is going to show you the full LSDB for this specific router. Since LSDBs must synchronize across routers in the same area, the LSDB should match all neighboring LSDBs for the given area.



Here you can see the following LSAs:

  • Type 1 (Router Link State) – Routers within the local area. If a DR/BDR is elected you will only see these advertised from the DR and BDR. 
  • Type 2 (Net Link States) – Routes within the local area. Same thing as Type 1s if a DR/BDR is elected you will only see these advertised from the DR and BDR. 
  • Type 3 (Summary Net Link States) – Routes generated by ABRs of other areas. Advertisements consist of the routes within that specific area. Keep in mind this is not the full LSDB of that area but just what the ABR is configured to advertise.
  • Type 4 (Summary ASB Link States) – Link ID for each known ASBR. (not shown)
  • Type 5 (Type-5 AS External Link) – Routes redistributed into the OSPF domain via an ASBR. (not shown)

Lets do the same for OSPFv3 and IPv6 and see what we get.



Everything looks pretty much the same.

Notice the advertising (ADV) Router is still shown as an IPv4 address? Keep in mind that OSPFv3 uses a 32-bit IPv4 address as the router ID. Election of the router-id is still the same as OSPFv2

There are also two new LSA types shown in the LSDB

  • Type 8 (Link (Type-8) Link State) – shows all IPv6 interfaces (or links) participating in the area for each router within the area.
  • Type 9 (Inter Area Prefix Link States) – Similar to the type 8 LSA, shows all prefixes participating in the area for each router within the area.

Alright that is it for OSPF backbone and normal areas. Pretty straight forward and simple. Next up we jump into stub areas!!