OSPF ABR vs ASBR: Differences Insight

In OSPF (Open Shortest Path First) routing, understanding the differences between Area Border Routers (ABRs) and Autonomous System Boundary Routers (ASBRs) is crucial for effective network design and is taught in every IT infrastructure course.

ABR is a router that connects different OSPF areas, facilitating communication between them and the backbone area (Area 0). They summarize and distribute routing information, helping maintain the OSPF network's hierarchy and efficiency.

Today we will discuss what is the difference between Area border router (ABR) and Autonomous system border router (ASBR).

What are the Differences Between ABR and ASBR?

The ABR and ASBR comparison table below shows the key differences between Area Border Routers (ABR) and Autonomous System Boundary Routers (ASBRs).

Let us now look at ABR and ASBR in detail and understand their work.

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Area border router (ABR)

An area border router (ABR) is a router that connects one or more areas to the OSPF backbone. When a router is located near the border between one or more Open Shortest Path First (OSPF) areas, such type of router is known as Area border router (ABR).

It stores and operates different routing information related to the topology of the area to which it is attached or to the backbone, as ABR is the member of both the particular areas it is connected and to the main backbone.

To make a connection between the OSPF areas and backbone networks, ABR is used. You can define the area ID as a dotted decimal number (i.e., 0.0.0.0) or a standard number (i.e., 0).

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To show the path to all networks the ABR will only send one router i.e. the default gateway route outside the area. ABR doesn’t send full route updates to other routers which will be time time-consuming and complex process.

Given below is an example to check whether a router is an area border router or not.

Command: show ip protocols

Ex. As it is clear from the above diagram Router R2 is ABR

R2(config)#do show ip protocols

***IP Routing is NSF aware***

Routing Protocol is “OSPF”

Outing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Router ID 2.2.2.2

It is an area border router

Number of areas in this router is 2.2 normal 0 stub 0 nssa

Maximum path: 4

Routing for Network:

12.0.0.0 0.0.0.255 area 0

23.0.0.0 0.0.0.255 area 1

Routing Information Sources:

Gateway           Distance           Last update

1.1.1.1                110                   00:01:30

Distance: (default is 110)

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Autonomous system border router (ASBR)

Autonomous system border router (ASBR): A router that is usually connected through an exterior routing protocol like BGP to one or more logical entities (AS).

The major work of ABRs is to brief sub-networks found throughout the OSPF system. It stores many copies of its link-state database and provides the path to the router.

ASBR is a router that is attached to other OSPF areas, followed by other routing protocols like IS-IS, IGRP, BGP, EIGRP, RIP, and Static.

Given below is an example to check whether a router is an area border router or not Command: show ip protocols

Ex. As it is clear from the above diagram Router R3 is ASBR

R3(config)#router ospf 3

R3(config-router)#redistribute eigrp 1 subnets

R3(config-router)#ex

R3(config)#do show ip protocols

***IP Routing is NSF aware***

Routing Protocol is “ospf 3”

Outing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Router ID 3.3.3.3

It is an autonomous system boundary router

Redistributing External Routers from,

eigrp 1, includes subnets in redistribution

Number of areas in this router is 1.1 normal 0 stub 0 nssa

Maximum path: 4

Routing for Network:

23.0.0.0 0.0.0.255 area 1

Routing Information Sources:

Gateway           Distance           Last update

2.2.2.2                110                   00:09:52

Distance: (default is 110)

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Conclusion

In conclusion, Area Border Routers (ABRs) is a router that connects multiple OSPF areas, including at least one backbone area (Area 0). Autonomous System Boundary Routers (ASBRs) is a router that connects an OSPF network to other networks.

ABRs connect different OSPF areas, facilitating routing information exchange and generating summary LSAs to optimize inter-area communication. They also filter routes to enhance network performance.

Conversely, ASBRs connect OSPF networks to external routing domains, managing the import and export of routes between OSPF and other protocols. Understanding these roles is crucial for optimizing the OSPF network.