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There are several components in the OSPF network; one such component which is important for connectivity is Area border router or ABR. ABR are the routers which sit on the border between backbone and low-level areas and connected to backbone with at least one interface and other interface to connect to low level areas.
Routing is an important aspect of IT networking. There are several routing protocols for LAN and WAN communication at network layer or layer 4 of OSI model.
Efficient exchange of routing information in an autonomous system is achieved by using a network layer dynamic-state routing protocol Open shortest path (OSPF).
OPSF network is a hierarchical topology comprising a backbone area to which the entire network is connected.
In this article we will learn more in detail about Area border Router or ABR in OSPF networks, their characteristics, why we need them and how they function.
What is ABR (Area Border Router)
Hosted between the OSPF backbone and stub area is the Area border router or ABR to facilitate routing of data traffic between the OSPF backbone and other areas it is connected to. It is an essential component in OSPF hierarchical design which provides efficient routing, scalability in larger networks by logically segregating them into manageable areas.
Traffic passing the backbone must get into an area via the shortest route.
- In Figure 1 above the link from router D to router E in area 0.
- When D link will fail, its traffic to F link goes from link D to link G and link E to link F.
- Since D is an ABR for area 1, the traffic to router F is internal to area 1 and must remain in there only.
- Traffic will not flow from link D to link E and then to link F because link D-E is in area 0 only and not connected to area 1.
Which means you need to deploy 2 layer 3 interfaces between router D and router E in which 1st layer interface 3 will belong to (backbone 0), and second layer interface on both router D and router E will belong to area 1.
ABR interfaces more than one area in OSPF and summarizes routing information of one area and advertises it in another area. This way routers in different areas can identify their destinations in other areas without storing the routing information of the entire network.
Advantages and Disadvantages of ABR
Advantages
- Improves network stability, performance, and scalability by eliminating need to store routing information at one place
- Improves network reliability and convergence by isolating effect of topology changes in every area
- Enhancement network flexibility and security with establishing different kind of areas with distinct requirements
- OSPF hierarchical topology creates a simplified network architecture which is easy to manage
Disadvantages
- Complex to setup and additional overhead as it requires separate ABRs, LSAs for every area
- Dependency on backbone area for connectivity and availability
- Suboptimal paths or routing loops are introduced often due to route filtering and summarization
How does ABR work?
- ABRs use hello packets and database description packets to form adjacencies with different routers in other areas
- They receive link-state-advertisements (LSAs) from other area routers
- LSBDs are built for each of the areas based on LSAs received
- Separate Shortest Path (SPTs) is calculated by ABRs for each area with Dijkstra set of rules
- For each area ABR generates type 3 summary LSAs based on SPTs. The summarized LSAs have summarized and aggregated routes having costs summing up intra-area cost and the inter-area cost.
- Using hyperlink state updates and link state acknowledgements ABRs flood summary LSAs to other routers in all areas
- 5 external LSAs for each area are generated by ABR basis external routes learned from static and dynamic routes.
- Using link state update packets and link state acknowledgments ABR floods external LSAs to other routers in all areas
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