For one, this is your host Habib's career. And we are now moving into lesson five of OSPF. And just to recap, in Lesson One, we have already discussed how to how to how to configure OSPF in point to point configuration. We also covered lesson two in lesson two we did the OSPF point to multipoint non broadcast networks and it had part one and part two. And we completed that we also moved to to to a lab which was related to OSPF authentication and and today we are going to discuss lesson five. But before I move forward I would I would like to advise you to go back and look at lesson one, lesson two, lesson three, lesson four, just to get better understanding and to catch up to lesson five.

So let's start. Now OSPF has a hierarchical structure. And the reason for this is for its scalability and it has two level of hierarchy. One is the backbone area, which is always known as the transit area, as well as the area zero. And the other areas are, are actually non backbone area. That's what they what they're known for.

So, the main purpose of the backbone area, which is it Zero, it basically. It, it allows, it allows to connect to other areas, which are non backbone areas. That's the first thing. And users don't usually connect to area zero. So those are the two things you have to remember about area zero. It should connect to all non backbone areas and the end users should not be connected to this area.

Because it's a transit area, usually it's the backbone for the band in a network environment. So now area one, our area two or the non backbone areas, there could be many areas right. So the intention of these areas is to connect the users to the network resources. The traffic between the non backbone areas As must always pass through the backbone area right. So, with this point, we have covered OSPF point to multipoint non broadcast network lab. So, when you do dmvpn usually you can you can, you can do point to multipoint GRP tunneling and you can pass the area zero totally and directly connect to other areas.

So, that's what it is. This topology I'm showing now in the in the in the slide here has also different role different routers that have different roles. So for example, if I if I come to this to this Which is area zero. You can see I have a router here which is called a SBR. Now SBR is a router that has one interface connected to the OSPF area and other interface it's connected to the external domain, which has external routes that are non OSPF. So that's the function of the SBR.

So if somebody asked what is an SPI router, you know it's it's a router that actually advertises external routes to the internal domain. Now the other the other routers are that are that are at the border areas are called Eb Rs. and not the ABR is a router that is connected to at least two different OSPF areas including the back end bone area abs are are very important basically because they contain the link state database information for each of the areas that they are serving. And and they advertise those routing those routing information between the areas so because they act as in they come in the middle of two different areas. So in this topology you have a br here between area one and area zero and ABR here between area two and area zero. Now, there's also internal routers internal routers are basically the routers within one area like in this one this router.

This router are basically internal routers, this router this router our internal routers, so that Those are internal routers. Now, these are the backbone routers, one, two, and then this one is also a backbone router. So that's all about hierarchical, hierarchical structure of OSPF. I hope you have enjoyed and let's move on to the next topic. Thank you very much.