Question about OSPF

[Adam373]

Hi If there is a network has total 4 area's (area 0) + 3 more area's and every area have 5 routers connected to switch (BMA) even area 0 And by having routers connected to switch so now should every area even area 0 has DR router My question is: Does Dr Routers in every 3 other areas communicate throughout each other or throughout the Dr router in Area0(backbone area)

[Richard Burts]

There are things in your question that I do not understand. But some things are clear. You seem to assume that having a DR is related to an area. That is not correct. A DR relates to a network segment (a subnet). If a network segment is multi access (think Ethernet, Frame Relay multi access, etc) then each subnet will have its own DR. You suggest that each area might have 5 routers. But you do not tell us how many subnets are in the area. If we assume that each of the 5 routers has 3 connected subnets then there might be as many as 15 DR in that area (since each subnet will have its own DR)..

[MHM Cisco World]

You must notice that there is different between DR and ABR,

DR router elect in it domain as DR of that domain, 

ABR is router borader between area,

They are two different function.

It can be DR and ABR be same router or it can be DR of area x is different than ABR of area x.

What is perfect? For me making DR different than ABR is better because it reduces router ospf work, imaging same router DR receive and send update in same area also the ABR that send l3 to area0 that huge work in router.

This my view.

[Joseph W. Doherty]

As @Richard Burts notes, what you're describing is unclear as you don't describe how the different area routers, that physically connect to the same switch, logically connect to it.  Without that information, cannot say whether they are capable of any intercommunication.

I will note, for OSPF areas to share routing information, they need to do so across area zero.  However, it's possible a non-ABR can be connected to multiple areas will route between those areas, but such cross area routing would only be done on that router.

For example given R1 <area10> R2 <area11> R3, R2 would know how to reach anything in areas 10 or 11, but R1 would only know of area 10 and R3 would only know of area 11.

As Rick further describes, an OSPF DR concerns itself with multiple routers (within the same area and) sharing the same subnet.

[Spooster IT Services]

Hi Adam373 , 

- The no. of DR's does not depend upon the number of OSPF areas in the network , it depends upon the  no. of network segments , therefore there can be multiple DR's in an OSPF area . 

-  and for DR's of 2 different areas two communicate , an ABR would be used , ABR is area border router , which will act as a gateway or passage between two different areas, ABR's generate LSA 3 for prefix sharing between two different areas .