Orhan Ergun 5 Comments

OSPF ABR ( Area Border Router ) is the router that non backbone area routers use it as exit from their area. In the OSPF RFC it is stated that if one router has two interfaces on the two area it can be considered as an ABR.

But in the implementation , in order to have an ABR , router needs to have at least one interface is connected to area 0 which is backbone area.

OSPF_ABR

 

In the figure, R3 is the backbone router but not an ABR since it has only connection to the Area 0. In order to be an ABR , router needs to have one interface in Area 0 and other at least one interface in different area.

R2 and R6 has a connection to both Area1 and Area 0 thus they are ABR. Also R4 is an ABR since it has a connection to both Area 0 and Area 2.

ABR aggregates topology information from one area to another area. What does it mean ?

In the figure although R3 seems only one router in the backbone area, let’s consider we have other routers in the area 0 as well. Their connections to each other and metric informations is not sent to other areas by the ABR.

Instead , ABR only sends the reachability information. The routers in different areas only know that they can reach each other but with the information provided by the ABRs.

Thats why , if you design multi area OSPF network , you can’t have end to end visibility. A router can have topology information about its own area only,

 
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  • jsorres

    Hi, i think that the correct statement is :

    “In the figure, *R3* is the backbone router but not an ABR since it has only connection to the Area 0.”

    Thanks for your blog and keep writing ! πŸ™‚

    • Hi jsorres, Exactly ! You are careful, I fixed the typo. I will keep writing but you will keep reading and commenting okay ? πŸ™‚

      • jsorres

        Of course ! πŸ™‚

  • gaurav

    Hello Sir,
    One question, I find it hard to grasp how the two nssa abr routers signal each other on which one is going to translate the type 7 to type 5. How, they know if the other ( say who was elected a translator) is down now, and one has to take the role. I mean, signalling between the two for this purpose. How its done, its not the N/P bit I believe, as far as I know.

    I read the rfc, they said sth about a new bit, but I couldn’t get it.
    Thanks for you help. πŸ™‚
    Gaurav

    • Orhan Ergun

      Its an election between them, highest address will do the translation only, this concept is used in many places by the way. DF election, Translator election all are the same idea. You wanna prevent duplicate ( Think about E-VPN and BUM traffic , or PIM bidder DF election , many other) And as far as I know it is N/P bit but check Ciscopress troubleshooting book, it should be written there as I remember