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Why all areas must be connected to area 0 in OSPF

Why all areas must be connected to AREA 0 in OSPF ?

When you learn regarding OSPF the first concept which you come across is that all areas must be connected to AREA 0 or backbone area to transfer traffic from one non Backbone area to other non Backbone area. Like if suppose you have three areas in OSPF -  Area 0 , Area - 1 , Area - 2. There are two users one is in Area 1 and other is in Area 2. They want to communicate with each other.Now how will they communicate ?  :roll:

User 1 of Area 1 send traffic to Area 0 then traffic will move from Area 0 to Area 2 and finally reach to user 2 of Area 2 right ? This a strict rule on which OSPF works. But why this rule exists? This post describes all about this.

As you know Routers in same area exchange each and every information known as LSA (Link state advertisement) to each other.Only summary routes or Summary LSA are forwarded to backbone area via ABR (Area Border Router) which is connected to this area and backbone area. Ok, here I think I need a diagram to explain this scenario so that I can make you understand more :

Here :

R1 - Backbone Router

R2 and R3 - ABR (Area border Router)

R4 , R5 , R6 , R7 - Area Router

Ok let's begin. In above topology R2 , R4 and R5 routers are in the same AREA . Yes! R2 is ABR and that's why it is in two areas at the same time. It is in Area 0 as well as Area 1. All these routers will send all types of the update to all routers in that area that is Area 1. R2 than convert all these updates or routes to summary routes and send this to R1 backbone Router. Same applies to Area 2.

R2, R4 and R5 will not see beyond R2 and they depend on upon R2 for the routes which are beyond R2 and here they act as Distance Vector. You may be wonder to know that isn't it :roll:

So by this, we reach to a conclusion that in inter-area or routers in the same area are act like a Distance Vector. Distance Vector is vulnerable to ROUTING LOOPS. Traffic from one area can only reach another area through area 0 otherwise Routing Loop will  formed in the network because routers in the same Area has same picture of the network in that AREA only. It does not have any idea or picture of another Area. Here in our case Area 1 Routers does not have any idea related to Area 2. If we have very large network in which we have lots of areas and they are connected to each other than it is more vulnerable to routing loops.  OSPF avoids loops by mandating this rule that all areas must be connected to area 0 in OSPF.

Hope you find this post informative. Please share this post if you like it.  :-)


  1. Could you explain the OSPF advantages & what the algorithm ?

  2. Thanks for the comment. I will post regarding OSPF advantages and its algorithm in future post.

  3. advantages is that updates is limited by area, is better for memory of router because number of update is decrese

  4. here you mention one wrong thing.
    like u said that routers within area behaves like distance vector which wrong,
    routers within single area behaves as link state routing protocol not distance vector.

    please fix me if i am wrong.


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