OSPF Confusion

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HI.
I'm revising OSPF for CCNA Discovery 3 Test and am a little unclear about adjacencies.

I gather OSPF routers can't form adjacencies with routers on different subnets. Why would you have 2 or more routers on the same subnet anyway, other than for redundancy purposes?

Leaving that aside though, if I understand this. If there's 3 routers on a ethernet network hub / spoke topology.

A, B C.

B in the middle, with connection to WAN.

A has couple of networks off FA interfaces and a serial link to B on s0/0

C has a couple of networks of FA interfaces and connected to B via serial s0/0

B has 3 serial ports. 0/0 to A, 0/1 to C and 0/2 to the WAN.

Does this mean A and C's only neighbour relations are with B?

Could these 3 connections appropriately come under area 0?

If at some later point, a fourth router, D, is attached to router A and only A. So a stub network I suppose. Sharing a point to point link with A, on another interface, then A would have neighbour adjacencies with both D and B?

And, could that link between router A and D, be set as area 1?

If that's right, maybe I get it. I'd appreciate some feedback. Cheers.

Straylight.

 

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Yes.

Yes. An area is as big as you want it to be.

Yes.

Yes. An area is as big as you want it to be.

Keep in mind that all areas must directly connect to the backbone area, Area 0 (or contain a virtual link through a transit area that connects to Area 0).

 

Provided that OSPF is configured correctly, OSPF forms neighbour relationships only with the routers directly connected to it. So in your example B would have two neighbours A & C would only have one. For CCNA you only need to know about area 0 or the backbone. When you introduce D the neighbours you describe are correct. D could be on area 1 as all areas must connect to area 0 but you are moving past the scope of CCNA and looking at A being an area border router.

 

Thanks. As I was writing that stuff it became a bit clearer.