OSPF areas divide large networks into smaller, manageable segments to reduce topology change overhead and limit the scope of link-state advertisements. Area Border Routers connect these segments and support route summarization to keep routing tables lean across the broader network.
OSPF Areas
OSPF is meant to handle really large networks, but the problem is that when your networks get so large, it becomes really difficult to manage, and changes within the topology can cause a lot of traffic across all of our networks. So what we can do to compensate for this is break apart our really large topology into areas and manage these areas separately when it comes to OSPF.
This is an example of an enterprise topology that we may want to consider breaking down into several smaller areas. In this example right here we have a backbone area, and this would be our area zero, and then we have several side areas to this that are offshoots. So maybe we label this 10, 20, 30, or something; we come up with some area labeling for each one of these.
The advantage to this is it breaks apart these OSPF areas into smaller sections for better utilizing our resources. An example of this is that OSPF has a lot of information that it passes back and forth. We've talked about all of the packets that are passed back and forth, and these link state advertisements that get passed back and forth, and that all happens within an area. So a change here on this router doesn't affect all of the other areas, it would only affect this one area. Or I should say it may not affect the other areas, especially if you've got redundancy involved within your network.
So this simplifies this whole process of how it would recalculate things. It simplifies how many link state advertisements go out. It simplifies how big the database is within the area. So this can really help break these sections down into smaller, manageable sections when it comes to OSPF.
One of the things that allows this to work is these area border routers. The area border routers are the routers that straddle more than one area. So these routers right here are attached to two different areas. They have maybe some interfaces that are attached to one area, and then they have another interface that's attached to another area. So that would be what an area border router is.
There's some great things that can happen. They act as kind of a gateway between these different areas. So things like the link state advertisements that happen within one area, it doesn't necessarily need to make those changes and send those changes out to the other areas. It would just depend on what this router decided: if this is information that does need to be shared, if it does update its routing table and does need to be shared, or doesn't need to be shared. So that is the area border routers.
One of the advantages that can happen on these area border routers are summary routes. We would break out our networks into some sort of hierarchy. What do I mean by that? Here are some routers here, and let's say we have some LANs that are hanging off of this. Maybe there's a lot of LANs that are hanging off of this, and this one right here has a 10.1.1.0/24, this one's a 10.1.2.0/24, this one right here is 10.1.3.0/24, and this one's a 10.1.4.0/24, and you get the point. I would go through and label all this. Maybe something over here, all the ones that are hanging off of this one is a 10.2.1.0/24, the ones that are 10.3.1.0/24.
So essentially we've got this hierarchy where everything within here is the 10.1.0.0/16 range, so everything, all the networks that are within this area fall under here. Everything in this area right here is the 10.2.0.0/16 range, and everything in here is the 10.3.0.0/16 range.
What would happen is at these area border routers, that would get summarized. So rather than a routing table here that would have all of those different networks that are within there listed out, same thing with this area right here, in this area right here you would see all the 10.1.1, 10.1.2, 10.1.3, but at this area border router it would get summarized into this 10.1.0.0/16, and that's what would be sent out to these other routers here.
So now when there is an update, let's say one of these networks goes down, this link state advertisement goes down for this network right here that goes offline, this router no longer has access directly to this 10.1.4.0 network. Well, what would happen is this summary route is still good, it doesn't need to recommunicate that out to any of the other routers. So that's the advantage of the summary route.
This would be, in our example, one, two, three, four, five, six, seven, there's seven different networks, plus all of the links in between. So there could be 10, 20 different networks built behind here, but it just gets summarized into this one. So where we have 20 different entries into the routing tables of all these routers, there's only one; it gets summarized when it transitions through this area border router.
The recommendation is that this middle area is area zero, and then it doesn't really matter what we call these other areas. Maybe we say this is area one, area two, area three, which, if I use the 10.1.0.0/16 example right here, I do like things when they line up really well like that, so this would be area two for the 10.2.0.0. But this also could be area 10, this area 20, or whatever you decide from that perspective.
Where you have more simplistic networks, like the topology we have, and it's just a single area OSPF, what they say is that this area should be area zero. So we would use area zero, since we're not going to break this down into multiple areas.
TechKnowSurge builds IT and cybersecurity professionals through hands-on, concept-first training built around real understanding — not memorization. Free interactive tools, structured programs, and 25+ years of real-world experience, all in one place.
Explore free tools and programs →