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Asked: 2026-06-01T09:45:23+00:00

I think I understand OSPF – it is Link-State routing, with a domain split

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I think I understand OSPF – it is Link-State routing, with a domain split into different areas.

Having one area as the backbone obviously creates a hierarchy, but I thought that the point of hierarchies was to decrease the amount of information that each individual node stored, and I don’t understand how OSPF does that.

From what I understand (from Computer Networks, A Systems Approach), is that routers on the border to the backbone area (ABRs) collect routing information about their outer area, then advertise this to the routers in the backbone (with the simplified costs), and then this is forwarded out to all of the other areas.

So from that, we still have the same amount of information in every router (the next hop to get to every other router), its just that we’ve changed the costs, and forced all outer area communication to go through the backbone.

Is this true? If so, what is the point of splitting it up into domains. If not, could you please point out where I’ve gone wrong.

Thanks alot.

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1 Answer

  1. Editorial Team

    Every router in an OSPF area knows the entire topology for its area and runs the SPF algorithm to find the shortest paths. Inter-area routes appear to your routers as all originating from the ABR, so that the adjacent area’s topology does not have to be added to the OSPF database and be included in the SPF algorithm. The point isn’t to make the routing table smaller, it’s to make the OSPF database in each area simpler and therefore the SPF algorithm less CPU intensive to run.

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