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As we know, during OSPF configuration we use the area. Each area has its different number, while there is only one area (area 0) which creates communications between all other attached areas.

I am confused that as we are using the same configuration techniques over all of the areas, but I don't know why we use areas if we can direct connect all of the PCs.

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If you're asking specifically what Areas do in OSPF, there are many resources out there which have a wealth of information. These are just a few selections:

However, a good answer to "What is the advantage of OSPF Areas?", is this quote from the Cisco Design Technote, What are OSPF Areas:

An OSPF network can be divided into sub-domains called areas. An area is a logical collection of OSPF networks, routers, and links that have the same area identification. A router within an area must maintain a topological database for the area to which it belongs. The router doesn't have detailed information about network topology outside of its area, thereby reducing the size of its database.

In short OSPF Areas allow you to segment your network topology to enable greater scalability and maintain a greater level of control.


Edit: In response to your comment "How can OSPF Areas increase the speed of communication?":

By itself, using OSPF Areas doesn't increase the speed of communication. It removes roadblocks to communication/scalability.

Think of it like this (this isn't a perfect analogy, and over-simplifies the issue, but it should help give the general idea):

If a company takes hundreds of telephone calls each day, and has to send all of it's telephone calls to a single secretary, calls into the company will be slow. However if there is a central secretary who knows about many secretaries who all take calls for a different portion of the company, calls can more quickly be processed.

A similar issue arises when routing tables for any one device get too large/unmanageable. Splitting them into more manageable areas can help speed traffic along to it's destination.

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  • Now i got it, but still there is one thing that how can AREA increase the speed of communication? would you like to give me some more comments.
    – Fahim Shah
    Nov 15 '13 at 20:10
  • @FahimShah excellent question, in my mind area would only increase propagation delay. However you can introduce aggregation in area border, to mask updates between areas to reduce work in the network. My general rule of thumb, if you don't know for absolutely sure why, always keep your IGP flat. We have flat IGP of some 1500 nodes, no problems what so ever.
    – ytti
    Nov 16 '13 at 8:16
  • @Brett Lykins, Sir your example is quite explanatory, I got it well, thumbs up four your spirit and hardworking
    – Fahim Shah
    Nov 16 '13 at 20:55
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All routers within an OSPF area keep a link state database (note that this is completely separate from the main routing table), where they're aware of all other routers and their links within the area. Each router within the area builds a topology tree of the area, with shortest paths to all other links/routers with itself as the root. This last part is important.

When an area grows large, the link state database (the tree or topology) that each router must maintain also grows large. This means that it can become more and more intensive for the router to process link state (topology) changes as there are now a large number of entries in the link state database. The tree grows larger and is more difficult to "keep up" with as there become more and more branches/leaves of the tree. Something else to keep in mind is that as the area (network) grows larger, there is greater potential for link state changes, and thus a greater potential for recalculations of the link state database. While the details of which are somewhat "out of scope" of this answer, the OSPF link state update process is also relevant here.

Ultimately, as a single area grows larger and larger, the SPF recalculations themselves will take longer to complete, and you have more risk of those SPF recalculations happening due to various reasons - the moral of the story is your routers' CPUs will be sad.

The "advantage" of OSPF areas is that they provide a means to alleviate the demands placed on the routers if they were otherwise in a single area, by way of cutting down entries within the link state database and pushing responsibilities of the link state database maintenance to area border routers for their respective areas. It allows for a way to keep the tree size manageable.

Thorough thought and planning is mandatory for designing/implementing multi-area OSPF - there are a number of situations where poor design in multi-area OSPF can bite someone.

Using areas doesn't necessarily increase the "speed of communication" but it can have significant performance benefits (if done properly) to the routers in your OSPF network, especially if your network is very large, because their CPU's aren't having to work as hard.

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  • 3
    In all honesty modern routers have zero issues with area sizes with hundreds of routers in them. There is no 'significant' benefit at all. If anything, multiple areas make your topology more complicated
    – mellowd
    Nov 16 '13 at 15:46
  • @John Jenser , Thanks to take out too much information from your knowlege, and made me elaborate about the term "Area" in the OSPF Routing.
    – Fahim Shah
    Nov 16 '13 at 19:05
  • 3
    @mellowd there's still a scaling limit of a single area, it doesn't matter if there's hundreds or thousands of routers in a single area - there's going to be a point where reconsidering the use (or lack thereof) of areas makes sense in your design considering the scale, or other factors. Nov 17 '13 at 1:55
  • @mellowd, your comment is quite frankly limited to those devices which have dedicated different resources to the data plane and control plane. OSPF areas are quite useful if you're running VoIP on a CPU-based router like the Cisco 2800; otherwise every link flap or router crash in the entire network has the potential of disturbing your voice quality (due to the requisite SPF calculations required). The larger the topology, the more potential for disruption. Nov 17 '13 at 20:39
  • I don't know about that. When last did you test? I once put an old 1841 inside a networks with well over 2500 LSAs and I saw no detrimental affect on CPU usage or transit traffic flow when there was a change in the core network.
    – mellowd
    Nov 17 '13 at 22:14
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The advantages of an “area” in OSPF configuration are:

  1. smaller routing table (no summarization by default): fewer routing table entries as network addresses can be summarized between areas
  2. smaller link-state database (LSDB): fewer routers exchanging LSAs, minimizes processing and memory requirements
  3. less-frequent SPF algorithm calculations: localizes impact of a topology change within an area.
0

I've stumbled across another use for a second area. If you have your area 0 distributing a default route, and do not want this default route injected into your OSPF device that has its own gateway to the internet, and said device can not do inbound route filtering, you need to set up a new area that does not use default-information originate.

Otherwise when OSPF comes up on said device, it will learn the default route via OSPF which will over-ride it's own default route.

My "device" in this instance is a VPN gateway (steelconnect) that has its own WAN interface used to connect to the internet, and is learning my internal routes via OSPF from my layer 3 switch on its LAN interface.

What happens is that the device comes up, contacts the internet, and then as soon as OSPF finishes loading, it drops off the internet, as its learned OSPF routing gives it a default route via the LAN interface and associated firewalls which just won't work for its VPN termination.

Using a second OSPF area in a second OSPF process allows you to run that instance without distribution of the default route.

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