OSPF query

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Sep 11th, 2008
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Hi All,

I am new to OSPF.Can somebody tell me how OSPF multiple area concept works. As per the theory, multiple areas wont be having identical database. Each area will have a locally significant database giving more importance for local area networks. But when I connected 2 areas( say 1, 2) and connected them using area 0 I can see the routes of area 1 in area 2 . How is this possible? Will the area continue to have the routes of the other areas, but only difference is in LSA received ?


Thanks in Advance

Raj

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Richard Burts Thu, 09/11/2008 - 19:08
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Raj


Perhaps we need to refine your understanding of how the OSPF database works and of the different kinds of OSPF routes.


The foundation of this understanding are the LSA types. While there are quite a number of LSA types for this discussion we need to understand only type 1, type 2, and type 3.


Type 1 and type 2 LSAs are generated by routers within the local area. Type 1 and type 2 LSAs are used in the calculation of the Dijkstra (Shortest Path First) algorithm. And the result of the calculation of type 1 and type 2 are the intra area routes which in the routing table show up as O routes.


Type 3 LSAs show what is advertised from other areas (the result of type 1 and type 2 of the other area). And type 3 LSAs result in inter area routes which in the routing table show up as O IA routes. An important aspect of type 3 LSA is that OSPF can process these and update the routing table without having to do a full SPF calculation.


So in your difference you may have area 0 which connects area 1 and area 2. For a router in area 1 its database will have LSA type 1 and 2 for the local routes. And any time that a link in area 1 changes state then the router must calculate the SPF to produce type O routes. But changes in link state in area 2 will come into area 1 as type 3 LSA and will be in the routing table as O IA routes. And if a link changes state in area 2 the router in area 1 will not be required to do a full calculation of SPF.


HTH


Rick

nik876159 Thu, 09/11/2008 - 21:26
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Rick,


I can brief it like this, Area 1 will be having only a summary LSA( LSA 3) from the other areas. My doubt is regarding the summary LSA. If I have got multiple networks in Area 2,192.168.1.0/24,192.168.2.0/24 and if I haven't summary address for the area, will each routes be published to the next area.How does the route information propagate in LSA 3

Thanks

Raj

paul.matthews Thu, 09/11/2008 - 22:38
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The LSA 3 may be a summary of the entire area, or there may be multiple LSAs for each network within the area.

paul.matthews Thu, 09/11/2008 - 22:26
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Simple answer - yes.


Right a little more info and apologies if I go into granny and egg sucking mode.


OSPF is a link state protocol. That means it knows about every router and every link within the area. Knowing that info lets each router build a picture of the area.


That would mean a typical enterprise network would have a large database in every router if it was in a single area, so there is the concept of areas. Each router within the area now only has the full picture of its own area.


We then bring in the concept of area border routers. These sit on the border of two areas (one of which is always zero) and they have the full picture of both areas.


As they are the route into or out of the area, the routers either side do not need to know the detail of what is in the other area, simply what is available and that is it via the ABR - inter area routes are getting rather close to distance vector behaviour.


So if you have three areas, 0,1 and 2. The ABR between 0 and 1 will advertise the contents of area 1 into area 0 as summary routes. those will be seen for the ABR between 0 and 2 and thus advertised into area 2. The same happens the other way.


That is with standard areas. There are variants of stub areas where the whole aim is to minimise the info held by routers within the area. If you have only one ABR (or don't mint the option for slightly less optimal routing), why list lots of routes available via it? originate a default and send that into the area.


The whole point is to have everything within the OSPF domain accessible from everywhere, with the best possible routing, while reducing excessive demands on router memory and CPU.


Hope that helps,

Paul.

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