ABR vs ASBR in OSPF

ABR vs ASBR in OSPF. If you are new to Network Engineering and you are learning Dynamic Routing Protocol from scratch, you want to understand the differences between ABR vs ASBR and if there are similarities you would like to learn those too. In this post, we will learn both similarities and differences. Let's first understand both of these terms. ABR is purely an OSPF terminology, but ASBR is not. In fact, the detailed post about ASBR and the usage of ASBR in Different Places of Networking is explained in our What is ASBR Blog post. ABR - Area Border Router is a device which is connecting two different OSPF Areas. One of those OSPF areas has to be Area 0, which is also known as Backbone Area.


  In the above topology, R3 is an ABR, connecting Area 0 and Area 1, R4 is an ABR as well, connecting Area 0 and Area 2. R1 is referred to as Internal Backbone Roter as it doesn't have any other connection than Area 0, Backbone Area. In this topology, there is also an ASBR - Autonomous System Boundary Router. It is called ASBR because on that router external prefixes are injected into the Internal network.

Can ABR be used as an ASBR?

  On the above topology, as I indicated above, R3 and R4 are the ABR routers. Can we use ABR as an ASBR?. So, can we inject prefixes from the outside networks to our Internal network? Yes, we can. We can inject (Redistribute) prefixes on R3 and R4, ABR Routers to the Internal Network. Thus, similarity point of view, ABR and ASBR functions can be on the same device. The difference is, when you hear the ASBR term, that is a device you inject prefixes from other networks to your network and. ABR is a device that is connecting two Internal OSPF Areas. Hope it is very clear now.

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Orhan Ergun

Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021

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