OSPF is a widely used routing protocol due to its efficiency and scalability. Within OSPF, specialized routers play crucial roles in managing network paths and ensuring optimal routing. Two of these key routers are the Area Border Router (ABR) and the Autonomous System Boundary Router (ASBR).
OSPF Area Border Router (ABR) is a router that connects different OSPF areas, facilitating communication between them and the backbone area (Area 0). OSPF Autonomous System Boundary Router (ASBR) facilitates the routing of data between an OSPF network and external networks.
In this article, we have discussed the Area Border Router and Autonomous System Boundary Router in OSPF, highlighting the differences between them. Furthermore, you can check out IT infrastructure courses to learn more technical details, such as configurations.
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.
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 a member of both the particular areas it is connected and the main backbone.
To make a connection between the OSPF areas and backbone networks, an 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).
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 an ABR
***IP Routing is NSF aware***
The Routing Protocol is “OSPF”
The outing update filter list for all interfaces is not set
The 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): 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 an ASBR
***IP Routing is NSF aware***
Routing Protocol is “OSPF 3”
The outing update filter list for all interfaces is not set
The 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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The ABR and ASBR comparison table below shows the key differences between Area Border Routers (ABRs) and Autonomous System Boundary Routers (ASBRs).
Feature | OSPF ABR (Area Border Router) | OSPF ASBR (Autonomous System Boundary Router) |
---|---|---|
Function | Connects OSPF areas to the backbone (Area 0) and manages intra-area routing. | Connects OSPF to external networks and manages inter-domain routing. |
Routing Information | Summarizes and distributes routing information between OSPF areas. | Imports and exports routing information between OSPF and non-OSPF networks. |
LSA Types | Generates Type 3 LSAs (Summary LSAs) for inter-area routing. | Generates Type 5 LSAs (External LSAs) for routes to external networks. |
Location in OSPF | Found within the OSPF domain, specifically at area boundaries. | Found at the edge of the OSPF domain, connecting to other routing protocols or the Internet. |
Role in OSPF | Maintains OSPF hierarchy and reduces routing table size through summarization. | Facilitates communication between OSPF and external routing protocols, enabling broader connectivity. |
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) 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.