OSPF area concept confusion

[joel75941]

Hello all,

I'm having trouble understanding some core concepts of OSPF.  Hoping someone can shed some light.  Please see attached. 

In the RED box:  OSPF multiple area concept states OSPF areas should be limited to routers being in the same subnet to form neighbor relationships.  Different subnet?  Different area.  Simple enough. 

In the GREEN box:  In the next chapter with ospf implementation, how can all 4 routers be in the same area if the links between them are clearly all different subnets?  What am I not understanding here?  Thanks in advance.

[Reza Sharifi · Accepted solution]

Hi,

Just because you have different subnets, it does not mean each subnet has to be in a different area. An area can have one or multiple subnets in it.  You can have for example 10 routers connecting together using /30 subnets and all be in the same area. This is actually very common.

HTH

[joel75941]

OK!  Thank you. 

[joel75941]

EDIT:  I think I just misunderstood the original explanation. 

If I understand correctly, the subnet doesn't matter when neighbors are in discovery.  Having routers in all different networks isn't bad design if it's all added to the same area.  The only reason to really split areas is to improve convergence in big infrastructures. 

I hope I got that right.

[Reza Sharifi]

Correct. Is addition to improving convergence, depending on the area, you can for example have a default route in one area and don't need to have and see all the routes that you usually see on area 0 (backbone).  So, smaller database and routing table. 

HTH

[Joseph W. Doherty]

Just to add to what Reza has already posted.

OSPF adjacent neighbors must "share" a common network. That network would be in just one area. The same router, though, can connect to multiple networks, and each of those networks can be in the same or different areas. (The latter making the router an ABR, as seen in your "red" box's D1 and D2 routers although not all the network prefixes are shown.)

Also, yes, as both you and Reza have noted, OSPF areas are to deal with large topologies, especially dealing with OSPF's usage of the Dijkstra algorithm.