OSPF muti-area advantage

Answered Question
Feb 26th, 2007

What is the advantage of implementing muti-area in ospf ? If no summray route is configured in ABR to reduce the size of routing table, I can't see any more advantage can have ? Anyone can tell me ?

I have this problem too.
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Correct Answer by royalblues about 9 years 7 months ago

Yes.

In case of a multiaccess network the router would update the DR & BDR and the DR inturn would flood this inframtion on the network.

HTH, rate if it does

Narayan

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Jon Marshall Mon, 02/26/2007 - 00:44

Hi

It depends on the number of routers you have that are running OSPF. Type 1 & 2 LSA's are contained within an area. If you have a lot of routers all in one area then each change needs to be flooded to all other routers within that area.

Area's are a good way to reduce the load on routers, reduce the size of the link-state database and contain the amount of LSA's needed within that area.

HTH

Jon

acbenny Mon, 02/26/2007 - 00:52

Hi Jon,

Is it means that once the network is changes, the router will update DR, BDR, and then DR will update all router within the same ospf area ?

Correct Answer
royalblues Mon, 02/26/2007 - 00:59

Yes.

In case of a multiaccess network the router would update the DR & BDR and the DR inturn would flood this inframtion on the network.

HTH, rate if it does

Narayan

hoogen_82 Mon, 02/26/2007 - 01:43

To be more specific, Once the DR and BDR have been elected, the other routers (known as DRothers) will establish adjacencies with the DR and BDR only. All routers continue to multicast Hellos to the AllSPFRouters

address 224.0.0.5 so that they can track neighbors, but DRothers multicast update packets to the AllDRouters address 224.0.0.6. Only the DR and BDR will listen to this address; in turn, the DR will flood the updates to the DRothers on 224.0.0.5.

And regarding your original question of using multi area it is because An OSPF routing domain can be split into several subdomains, called areas, which limit the scope of LSA floodig. A router having attachments to multiple areas is called an "area border router" (ABR). The primary function of an ABR is to provide its attached areas with Type-3 and Type-4 LSAs, which are used for describing routes and AS boundary routers (ASBRs) in other areas, as well as to perform actual inter-area routing.

Cheers

Hoogen

Jon Marshall Mon, 02/26/2007 - 01:46

Hi

As Narayan said yes it does.. But also areas can help in NBMA environments as well ie a good example would be if you had a point to multipoint WAN configuration. You would generally want this in it's own area so if there are changes in any other area you do not flood your wan links with unnecessary updates.

HTH

Jon

Richard Burts Mon, 02/26/2007 - 07:05

There is another advantage of multi area OSPF that has not been clearly described in this thread. Receipt of LSA type 1 or 2 causes full calculation of the Link State Algorithm (Dijkstra) but receipt of LSA type 3 does not cause full calculation of the Link State Algorithm. So if it were single area OSPF any link state change on any router would force a full calculation on all routers. But in multi area OSPF a router only does a full calculation when there is a link state change within its own area but not a full calculation when there is a link state change in a different area. As the network gets larger this becomes a significant performance difference.

HTH

Rick

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