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ospf area

Why does OSPF insist on all inter area traffic passing through the backbone


Re: ospf area

Hi Suryakant,

When you design an OSPF network, OSPF inherently contains some kind of hierarchy.

The top of the hierarchy is the backbone (area 0).

The leaves of the hierarchy are the rest of the OSPF areas.

This architecture has several benefits:

- you can design a hierarchical addressing scheme

- you can do summarization of addresses on the ABRs (thereby reducing the size of ospf databases and routing tables and containing some SPF calculations within areas)

- routing updates between the backbone area and leaf areas flow through the ABRs.

You can do update filtering on the ABRs to control the flow of updates from the backbone to the leaf areas and from the leaf areas to the backbone:

As far as your all updates flow through the ABRs, (from the backbone area to the leaf areas and vice versa), of course, traffic will pass in the reverse direction, through the ABRs, from the leaf areas to the backbone area, and depending on its destination, to another leaf area.

- As from the above follows: you can create ordered and controllable traffic patterns by traffic flowing from the leaf areas through the backbone to other leaf areas (this is called interarea traffic passing through the backbone)

Of course, ABRs and backbone routers generally have to be higher class routers because they deal with much more updates and traffic than routers in the leaf areas.



Re: ospf area

Hi suryakant,

According to OSPF structure, Area 0 is the Backbone Area, All areas must be connected to Area 0.

OSPF has different LSA types, those LSAs are needed to build every OSPF router Link state database.

The Inter Area LSA represent type 3 LSAs , Type 3 LSA is just Type 1 and Type 2 LSA converted to type 3 LSA at the border router. So the Border router which creates this LSA , therfore , it has to pass the backbone Area as indicated above the backbone is needed to connect all Areas.