Today I am going to begin to talk about OSPF and OSPF areas. OSPF is short for Open Shortest Path First, and is an IGP link-state protocol. OSPF operates with routers all having the same identical topology map and from that identical map, every router calculates the shortest path possible using Dijkstra’s algorithm. The shortest path is calculated by the metric cost (cost = reference bandwidth / interface bandwidth). Once the topology is fully up, routers flood updates (LSAs) whenever something in the topology changes like a link going down or a router being added to the topology.
Area- Today we are going to dive into OSPF areas and how routing works between them. OSPF is an IGP which means that the routing is done within a single AS (Autonomous System.) An OSPF area is a logical grouping or subdivision of networks and routers within this single AS. It is like taking a single pie of pizza, and cutting it into slices with each area being one slice of the whole pie. There are many reasons for dividing an OSPF AS into multiple areas but one of the main reasons is to limit LSA flooding, and SPF calculations. Imagine an OSPF single area for a very large organization with over 200 routers, as you can tell having a single area would increase processing power by a lot and even slow down the convergence time. OSPF areas make it much more realistic to scale while allowing it to be still manageable.
Backbone Area- An area that is a mandatory area no matter what in an OSPF topology is a backbone area. A backbone area can also be recognized as area 0, and only Area 0 can represent the backbone area. The backbone area is the core area that every LSA or route from one area must pass through first before it is allowed to traverse to another area. If there are multiple areas in an OSPF topology, the ABR (Area Border Router) must have at least one interface in the backbone area either directly connected or via virtual link (virtual links are only temporary workarounds not a standard design). An ABR sits at the boundary between areas maintaining separate LSDBs for every area connected to one of its interfaces. This is a purposeful hierarchical design by OSPF to prevent routing loops and enable inter-area routing. Without a backbone area in OSPF, every different OSPF area inside an AS would be isolated because OSPF does not allow direct non-backbone area to non-backbone area communication by design.
Intra-area- When it comes to routing in OSPF there is either intra-area routing or inter-area routing, first let’s talk about intra-area routing. Intra-area routing is a type of route that is confined within a single OSPF area. Intra-area routes do not cross any boundaries and if an ABR is a part of the topology, there can be routes that don’t even go to the ABR because the routing is purely internal. These routes are the most preferred routes because they are formed with very detailed LSAs (1 and 2). The routes that are installed into the routing table were calculated with information from Type 1 and 2 LSAs. Type 1 and 2 LSAs are the most detailed LSAs which is why they are more preferred than inter-area routes. On the routing table, you will see these routes represented by “O.” So if you’re ever configuring OSPF in a lab or a real environment when you see O in front of the route, it is for certain that it is an intra-area route.
Inter-area- Inter-area routes on the other hand are routes that originate from one OSPF area, with the destination being in a completely different OSPF area. These routes are less preferred than intra-area routes because they do not use Type 1 or 2 LSAs to build the route, instead the ABR makes a summary LSA after the intra area routes are created. Once the Type 3 Summary LSA is made by the ABR, that Type 3 LSA then gets flooded in the backbone area so every ABR can receive them. Once every area has received a Type 3 LSA, inter-area routes are formed. Area 1 can now communicate with Area 4, 5, or 6. Unlike intra-area routes they are represented on the routing table with this symbol before the route entry “O IA.” Every time you see O IA, it is certain that it is an inter-area route formed with Type 3 summary LSAs. Another caveat as I stated in the above paragraphs, any inter-area route must traverse the backbone area first. So if area 1 wants to send a packet to area 2, the packet would go to the ABR, exit out of the area 0 interface, then be routed to the destination ABR area 0 interface, which is then forwarded to the destination area.
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