Can OSPF be accessed without Area 0 , Technical Reason of Area 0

[Ibrahim Jamil]

Hi Freinds

Can OSPF be accessed without Area 0 , what is the Technical Reason of Area 0 Besides its the Backbone of the OSPF Routing Domain?

Thanks

[Richard Burts]

If your OSPF network has only a single area then it does not need to have area 0, it could be any number. But if there is more than one area then there must be an area 0.

Communication between areas is done with Area Border Router. And by definition an ABR has an interface in area 0. If you had a router with multiple areas but no area 0 it would not function as ABR. For example a router with connections in area 10 and area 20 would know the routes of both areas. But it would not advertise area 10 subnets to area 20 and not advertise area 20 subnets to area 10.

[M02@rt37]

hello @Ibrahim Jamil,

There is only one way to use OSPF without an Area 0, and that is to use OSPF with a single area. If you only have one OSPF area, you can number it any way you like, but if you have even two areas, you must have an Area 0. OSPF can work efficiently without area 0 when there is only one area.

Technical reason for Area 0 when we have multiple areas?

OSPF is designed with a hierarchical structure to enhance scalability and ease of management. The use of multiple areas allows for a modular approach to network design. Area 0 acts as the central point for interconnecting these areas, providing a scalable and modular architecture.

SPF (Shortest Path First) tree calculation is performed for each OSPF area individually. By having Area 0 as the backbone, SPF calculations are reduced. Routers within individual areas only need to calculate SPF trees for their local area, while routers in Area 0 calculate SPF trees for both Area 0 and other connected areas.

Area 0 provides a common connectivity point for all OSPF areas. It ensures that any area, even if it is not directly connected to other areas, can reach all other areas through the backbone. This is essential for building a connected OSPF network.

Inter-area and external route exchange occur through Area 0. Routes from one area to another and routes redistributed into OSPF are exchanged through Area 0. This centralized point of exchange simplifies routing table calculations and improves overall network efficiency.

[balaji.bandi]

As mentioned Area 0 is required if you have more areas in the network, the areas are divided to make simplified routing.

follow below thread or google for RFC more details.

https://community.cisco.com/t5/routing/why-are-we-required-area-0-in-ospf-interview-question/td-p/4797501

[MHM Cisco World]

OSPFv2 introduced areas as a way to cut down on the size of the link-state database, the amount of information flooded, and the time it takes to run the SPF algorithm, at least on areas other than the special backbone area.
we need Area 0 to interconnect these area.

[Joseph W. Doherty]

"Can OSPF be accessed without Area 0"

Yes, but there are limitations, many described in prior other posters replies.

"what is the Technical Reason of Area 0 Besides its the Backbone of the OSPF Routing Domain?"

Scalability!!!

As also described in other posters prior replies, the SPF scalability is a major concern, but ABRs can also be used to limit routes crossing an ABR.  Stub area ABRs filter routes entering a non-zero area, while an ABR can summarize routes being sent into area zero.  (Route summarization is often key to scalability, and just not for OSPF.)

BTW, the other posters' replies might make it seem you cannot communicate between non-zero areas without having an area zero.  Well, it's possible, but as @Richard Burts described, non-zero areas don't share each other's routes (by default) as an ABR would, but two OSPF options come to my mind to circumvent this limitation (neither of which I would recommend for a "real" network in preference to using an ABR.  [BTW, in the example Rick described, a router with two non-zero areas, it could be changed to an ABR by placing a loopback interface into area zero.])

[Ibrahim Jamil]

Thanks all for ur informative Answers

[Isabella54]

In OSPF, Area 0 is essential as it serves as the backbone of the routing domain. The technical reason is that OSPF requires a common area (Area 0) for interconnecting different OSPF areas. It facilitates communication between areas and prevents routing inconsistencies. Without Area 0, OSPF lacks a central point for route exchange, impeding proper functioning and segmentation within the OSPF network.