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PyNet Labs- Network Automation Specialists

What is ASBR in networking?

Author : Pankaj Kumar
Last Modified: October 17, 2023 
What is ASBR Featured Image

Introduction

If you have knowledge regarding OSPF, you already know that it is a routing protocol that is mainly used to divide a network into different areas. Each area has its separate topology as well as the information regarding the routing. This makes it less complicated and also results in less overhead. If someone wants to connect their OSPF network with a different network that uses different protocols like EIGRP, BGP, and many others. How can one exchange the routes between these networks?

That’s where ASBR comes into action. In this blog, we will explain what ASBR in networking is, its purpose, and how it functions, along with its advantages and disadvantages.

Before getting into more details, let’s first understand what ASBR is.

What is ASBR in Networking?

ASBR stands for Autonomous System Boundary Router. It is a router that runs multiple protocols and serves as a gateway to routers outside the OSPF domain and those operating with specific protocols such as EIGRP, BGP, and many others. The ASBR is capable of importing and translating different protocol routes into OSPF through a method called redistribution. Route Redistribution allows the ASBR to advertise external routes to the OSPF network and vice versa.

ASBR in Networking

For example, when you have an OSPF network that connects to another OSPF network that belongs to a different autonomous system (AS). An AS is a group of networks that tends to share the same routing policy and administration. In this case, you may want an ASBR to redistribute routes among the 2 OSPF networks, given that they’re considered external to each other.

Purpose of Autonomous System Boundary Router

The purpose of ASBR is to permit communication among different autonomous systems or routing domains. An autonomous system is a group of networks underneath a common administration that share a common routing policy.

For instance, an ISP may additionally have its personal autonomous system that uses BGP in order to communicate with other ISPs, and its clients use OSPF to communicate within their very own networks. An ASBR can connect these two autonomous systems and allow them to exchange routes.

Hence, ASBR is of great importance in filling the communication gap between two different areas using different protocols.

We now have a clear understanding of what ASBR in OSPF is and its purpose. Let’s move on to the functioning of ASBR.

How does ASBR function?

An ASBR functions by running a couple of routing protocols on its interfaces and performing redistribution among them. For instance, if an ASBR has an interface connected to an OSPF area, it’ll run OSPF on that interface and participate in the OSPF process. If it has another interface connected to a non-OSPF network, it’ll run any other protocol on that interface, such as with EIGRP, BGP, or static routes.

The ASBR will then redistribute routes from one protocol to any other, primarily based on its configuration. For instance, if the ASBR desires to redistribute EIGRP routes into OSPF, it will take the EIGRP routes from its routing table and inject them into the OSPF LSDB (link-state database) as external LSAs (link-state advertisements). These external LSAs will then be flooded to all OSPF routers within the same area because of the ASBR.

Similarly, if the ASBR desires to redistribute OSPF routes into EIGRP, it will take the OSPF routes from its routing table and inject them into the EIGRP topology table as external routes. These external routes will then be advertised to all EIGRP neighbors of the ASBR.

The ASBR can also filter or summarize the routes it redistributes based totally on its configuration. For instance, if the ASBR desires to redistribute only particular EIGRP routes into OSPF, it can use an access list or a prefix list to match those routes and deny others. If the ASBR desires to redistribute only a summary of OSPF routes into EIGRP, it can use a summary-address command to create a single combination route for multiple subnets.

We have explained the functioning of ASBR in networking. Let’s now discuss its advantages and disadvantages.

Advantages of Autonomous System Boundary Router

Some of the advantages of using an ASBR are:

  • It gives connectivity and route exchange between networks using different routing protocols.
  • It allows for flexibility and scalability in network layout and expansion.
  • It enables route optimization and load balancing by way of deciding on the best path among a couple of paths.
  • It helps policy-based routing by using as well as applying filters or attributes to the redistributed routes.

Disadvantages of Autonomous System Boundary Router

Apart from all the advantages, there are also some disadvantages to using ASBR. These are:

  • It increases the complexity and overhead of network configuration and management.
  • It introduces potential routing loops and inconsistencies because of redistribution.
  • It may cause suboptimal routing paths and increase latency due to route translation.

These are the advantages and disadvantages related to Autonomous System Boundary Router.

Frequently Asked Questions

Q1 – What is ASBR used for?

ASBR, or autonomous system boundary router, is used to exchange routing information with routers that belong to different areas using different routing protocols.

Q2 – What is ASBR in BGP?

ASBR is a router that is usually connected to various autonomous systems via multiple protocols. In BGP, ASBRs use MP-BGP to advertise labeled VPN routes between ASes.

Q3 – How to identify ASBR in OSPF?

One can quickly identify the ASBR in OSPF. If a router has one interface in the OSPF domain and another in any other routing protocol domain (e.g., RIP or EIGRP), it is considered to be an ASBR.

Q4 – What is the role of ABR in OSPF?

ABR stands for Area Border Router. It connects different OSPF areas and advertises summary routes to them.

Conclusion

In this blog post, we have learned about ASBR in networking. We have seen that an ASBR is a router that connects different routing protocols and autonomous systems using redistribution and type 5 LSAs. We have also discussed the purpose, function, advantages, and disadvantages of using an ASBR.

We hope that this blog post has helped you understand what is ASBR in networking and how it works.

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