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What is ABR in OSPF?

Author : Pankaj Kumar
Last Modified: February 7, 2024 
ABR in OSPF Featured Image

Introduction

OSPF, or Open Shortest Path First, is one of the popular routing protocols that mainly utilizes link-state information in order to calculate the best path for sending the data packets to the receiver end of a network. OSPF functions by dividing an extensive network into smaller areas, and these areas have their own topology and routing information. However, this will create new challenges, such as how one can share routing information between different areas without flooding the routers with too much data.

To overcome this problem, ABR or Area Border Router is used. ABR in OSPF is a router used primarily to connect OSPF areas and acts as a gateway between them. In this blog, we will explain what ABR is, its role, working, and its advantages and disadvantages.

Before getting into more details about area border router in OSPF, let’s first understand what ABR really is.

What is ABR in OSPF?

ABR is a router that has interfaces in more than one OSPF area. It is responsible for summarizing the routing information from one area and advertising it to another area. In this manner, the routers in different areas can find out about the destinations in other areas without storing the detailed topology of the whole network.

Let’s understand the ABR in OSPF with the help of an example. Assume we have a network with three OSPF areas: Area 0, Area 1, and Area 2. Area 0 is the backbone area that connects all other areas. Area 1 and Area 2 area have their own subnets and routers. The following diagram explains the way ABRs can connect specific regions:

Example of ABR

In this example, R1 and R2 are ABRs that connect Area 0 with Area 1 and Area 2, respectively. R1 summarizes the routing information from Area 1 and advertises it to Area 0 as a single route. Similarly, R2 summarizes the routing information from Area 2 and advertises it to Area 0 as a single route. RA and RB are internal routers that belong to one area each. They do not need to know the topology of different areas, only the summary routes provided by the ABRs.

Now that we have a better understanding of ABR in OSPF, let’s focus on how it really works.

How does ABR work?

An ABR works by keeping separate LSDBs for each area it belongs to and also exchanging routing information between them. The working of an ABR is discussed below –

  • The ABR forms adjacencies with different routers in every area with the help of using hello packets and database description packets.
  • The ABR receives link-state advertisements (LSAs) from other routers in every area, using link-state request packets and link-state update packets.
  • The ABR builds separate LSDBs for every area using the received LSAs.
  • The ABR calculates separate shortest path trees (SPTs) for each area using the Dijkstra set of rules.
  • The ABR generates type 3 summary LSAs for each area based on the SPTs of other areas. The summary LSAs incorporate the summarized or aggregated routes from other areas, with their costs calculated as the sum of the intra-area cost and the inter-area cost.
  • The ABR floods the summary LSAs to other routers in every area using hyperlink-state update packets and link-state acknowledgment packets.
  • The ABR generates type 4 ASBR-summary LSAs for each area based totally on the location of AS boundary routers (ASBRs) in different areas. The ASBR-summary LSAs include the router ID and cost of the ASBRs that inject external routes into the OSPF area.
  • The ABR floods the ASBR-summary LSAs to other routers in every area with the use of link-state update packets and link-state acknowledgment packets.
  • The ABR generates type 5 external LSAs for each area, based totally on the external routes learned from different routing protocols or static routes. The external LSAs comprise the destination and cost of the external routes and a flag that shows whether the route is type 1 or type 2.
  • The ABR floods the external LSAs to other routers in every area with the usage of link-state update packets and link-state acknowledgment packets.

We have explained the functioning of ABR in OSPF; let’s move on to the advantages and disadvantages of using ABR.

Advantages of Area Border Router

It offers various advantages, some of these are:

  • It reduces the quantity of routing information that needs to be exchanged and stored with the help of the routers in different areas, which improves the network scalability, stability, and overall performance.
  • It isolates the effect of topology changes within every area, which improves network convergence and reliability.
  • It allows OSPF to help different kinds of areas with distinct characteristics and requirements, enhancing network flexibility and security.
  • It offers a hierarchical structure for OSPF networks, which simplifies the network layout and management.

Disadvantages of Area Border Router

ABR also has some disadvantages, some of these are:

  • In order to function appropriately, ABR requires separate LSDBs, SPTs, and LSAs for each area, which can lead to an increase in the complexity as well as the overhead of the network.
  • Secondly, they depend on the connectivity and availability of the backbone area (also known as area 0).
  • With ABR, potential routing loops, as well as the suboptimal path, can be introduced. All this can be due to the summarizing and filtering routes between areas.

These are the advantages and disadvantages related to Area Border Router in OSPF.

Frequently Asked Questions

Q1 – What is an OSPF Asbr?

ASBR, or autonomous system boundary router, is a router that serves as a gateway for the routers that are not part of the OSPF domain or the routers outside the OSPF domain operating on different protocols.

Q2 – What does ABR mean Cisco?

ABR, or Area border router, is a router that is mainly used to connect multiple OSPF areas. It builds separate LSDB for each area and also calculates the best path to other areas.

Q3 – What is ASBR in routing?

ASBR stands for Autonomous System Boundary Router. It is a router that connects an OSPF network to another network using a different protocol.

Q4 – What is DR and BDR in OSPF?

DR and BDR are Designated Router and Backup Designated Router in OSPF. They are elected on multi-access networks to reduce routing overhead and optimize convergence.

Conclusion

ABR is a crucial component of OSPF that enables it to scale to large networks by dividing them into smaller areas. However, ABR also introduces some complexity, overhead, and challenges for OSPF networks. Therefore, network administrators should carefully plan and implement their ABRs and their functions to optimize their OSPF networks. In this blog, we have explained what area border router in OSPF and also how it works. We also have discussed the advantages and disadvantages associated with the utilization of ABR. You can checkout OSPF BGP Course to learn more about ABR.

If you have any questions or feedback, please leave a comment below. Thanks for reading!

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