![]() Standard Area – a nonbackbone area that must be connected to the backbone area it accepts intra-area, inter-area, external and a default route Regular (nonbackbone) areas can have several subtypes: standard area, stub area, totally stubby area, not-so-stubby area (NSSA), and totally stubby NSSA. Now that you know the basics necessary to fully understand OSPF areas, we can discuss about different OSPF area types. The same applies to O N1 and O N2 routes. ![]() Note: E1 are type 1 routes – used when there are multiple ASBRs advertising a route to the AS external cost is added to the internal cost of each link Į2 are type 2 routes – used if only one router is advertising a route to the AS external cost does not change External routes – routes from other autonomous system (AS), external to the particular OSPF area appear as O E1 or O E2 in a standard area and as O N1 or O N2 in a NSSA or totally NSSA area.Inter-area routes – routes originated in other OSPF areas marked with O IA.Intra-area routes – routes originated and learned in the same area, i.e.Default route – a route used to propagate traffic to an external area marked with O*IA.OSPF route – typical route marked with O in routing table indicates OSPF as the origin of the route.We also distinguish between different types of routes in routing tables: For example, if it is connected to the backbone and nonbackbone area, and to a non-OSPF network, it is both an ABR and an ASBR. Note: A router can be of more than one type simultaneously. Autonomous System Boundary Routers – have at least one interface connected to another autonomous system (such as another OSPF system or EIGRP).Area Border Routers – have interfaces attached to multiple areas, and connect nonbackbone areas to the backbone area.Backbone Routers – have at least one interface connected to backbone area.Internal Routers – have all of their interfaces in the same area, i.e.With respect to the area(s) they interconnect to, we differentiate between internal, backbone, ABR (Area Border Router) and ASBR (Autonomous System Boundary Router) OSPF routers. Also, users are not generally placed within a backbone, but within nonbackbone areas which will be explained in further text. So, the backbone area is the main area to which other areas connect, either physically or by using OSPF virtual links, to exchange and route information. The main difference between these two areas is that the backbone area represents the core of the network and provides the fast movement of network traffic, while the nonbackbone area connects users and resources. Except the fact it is a link-state routing protocol (it has a complete picture of the network routers send the state of their own interfaces/links to all other routers), which I am sure you all knew, OSPF implements a two-layer area hierarchy: the backbone area, also called OSPF area 0, and the regular ( nonbackbone) area. In order to understand OSPF areas, you should be aware of a few important things regarding the OSPF routing protocol. Even though I will give a brief introduction into some basic OSPF concepts, it is assumed you have a prior knowledge of the OSPF (CCNA level). In this article, I will explain different types of OSPF areas, and when should you consider dividing your network into areas. Also, dividing an OSPF network into areas makes it more manageable, especially if it gets larger. ![]() Using multiple OSPF areas reduces the frequency of SPF (Dijkstra) calculations, the size of routing tables, as well as LSU overhead, and therefore improves network performance. ![]() Whether you are preparing for the Cisco IP Routing exam or you are an experienced network engineer planning to implement the Open Shortest Path First (OSPF) routing protocol, you should familiarize yourself with the capability to divide your network into separate OSPF areas. ![]()
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