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Getting started with OSPF: OSPF areas

Hi all, I am back with yet another routing protocol. Now that we know what is Link State routing, let us now get acquainted with a routing protocol which is based on link state routing. Well, OSPF it is..!!

Open Shortest Path First –> Open as it is not proprietary and can be used on all the routers. Shortest Path First  because it uses Dijkstra’s Algorithm to find the best routes. This algorithm first finds the route with the smallest weight and then calculates routes to reach all the other routers. It basically calculates the SPF tree.

OSPF does not use a TCP/IP transport protocol but encapsulates its data in IP packet with protocol number 89. It, therefore, implements its own error detection and correction functions. OSPF uses multicast addresses (IPv4) and FF02::5 (IPv6).

OSPF is IGP (Interior Gateway Protocol), as it is used for routing within an  Autonomous System (AS). AS in IGP is quite different than from AS in BGP 😛 Here AS is a  group of all the routers, running same IGP protocol, like RIP, EIGRP, OSPF. Now, OSPF can be used in huge networks, with around 200 routers (advantage over RIP). You can imagine, that in such big networks, the routing table would exceed sizes and how difficult it would be to manage these many routers. That is why the AS in OSPF is again split into small groups called as area. All the routers in one area will now have the same LSDB (Link state database). They are not aware of the other areas. This reduces the size of the routing table and also, now it easy to debug and manage a small number of routers. You can see below that you and I fall in different areas. My area is bigger than yours..!!


Now, each router completes the puzzle of its area only ie. each router within the same area will have the same view of its area only. Then, how does data move from one area to another? Well, to solve this, OSPF introduced Area 0. Area  0 connects all the areas, you can consider it as the center and other areas connecting to it. Hence, all the data will follow from area 0 only.  Even if my area is bigger in size, it has no existence without area 0 😛


Area 0 is also called as Backbone area. All the routers in area 0 will have routes to reach the other areas. OSPF supports VLSM (Variable-length subnet masking), CIDR, therefore using summarization and aggregation size of the routing table for area 0 routers can be reduced.

Apart from backbone area, OSPF includes stub area and transit area. With the increasing  complexity of networks, these areas make it easier to route the traffic. But backbone area is the most important area.

I hope that  you are now familiar with areas of OSPF. In the upcoming posts, we shall get connected with further concepts of OSPF.

Till then, Happy networking with OSPF…!!!

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