OSPF design question

Unanswered Question
Nov 21st, 2007

Hello everybody

There is a network of 4 routers in OSPF area 0, where the routers are in a "ring" topology. System is configured in a way that all connections between routers are made within /30 subnets. Therefore, there are 4 segments, each of them with separate DR and BDR elected. My question is:

Is such design a good choice and what are the advantages of such design (if there are any) vs. design where all routers in the same Area would also be in the same segment/subnet (and then we would have a single DR/BDR in within area)?

Thank you!

I have this problem too.
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royalblues Thu, 11/22/2007 - 03:48

If all the router connections are /30 subnets, it makes mroe sense to configure the links as point to point. This configuration would not require any DR/BDR election at all. The DR/BDR concept is more relevant in a multiaccess network to reduce the number of adjacencies.

The traffic flow and pattern would dictate the number of routers within an area. It is perfectly possible to even have 20 routers in one area



zoran_brakus Fri, 11/23/2007 - 16:09


Thanks for your answer. I was also thinking about possibility to have a point-to-point links in such situation. But actually I'm here more interested to know about advantages/disadvantages of the current design vs. design with all routers in the same subnet. What would be your choice and why?

cisco_lad2004 Sun, 11/25/2007 - 05:08

Hi Zoran

DR reduces LSAs exchange, so each none DR only send to DR (instead of all OSPF routers).

there is also a need for BDR, then DR and BDR needs to be connected.

with only 4 routers in ur area, u are not really achieving much by using DR/BDR set up.

if u had a bigger number of routers, then there is a good argument for using DR/BDR. Here is the design can go either way, with perhaps a personal preference for P2P (just to avoid DR/BDR election process).



zoran_brakus Sun, 11/25/2007 - 09:04

Hi Sam

Thank you for your answer. I perfectly understand your point with P2P against existing DR/BDR in every /30 subnet.

However, I'm still not sure if, for example, putting these 4 routers in a single subnet and then having a single DR/BDR in the Area would bring something more (either positive or negative)? Maybe there are some other concerns besides LSAs exchange? Am I wrong if I see it also as a matter of scalability? What if I want to add more routers and if the topology becomes an "extended" ring (with n routers in a ring and then couple of additional routers connected aside)?



guruprasadr Sun, 11/25/2007 - 20:50


In addition to the replies of SAM and Narayan.

Each Area has an Unique DR and BDR. Different Area's are connected via Area Border Router (ABR) and different AS are connected via Autonomuous System Border Router (ASBR).

Having a Single Area eg: (normally with Single DR/BDR) is OK unless and until you Area has limited Routers.

Consider your Network is going to grow Huge means, with Single area for overall network is not advisable because the all Network Advertisements in terms of LSA's are propagated in the Area. Every Router will have routes to all Networks.

Its advisable to have multiple Area's based on the requirement and connect back to the Backbone Area

With "n" Routers in a Ring and Single Area will become Cumbersome in terms of Routing Table Size and others.

Refer link below for: CISCO OSPF Design Guide:



Best Regards,

Guru Prasad R

jocampo35116 Mon, 11/26/2007 - 00:17

By jocampo35116 :

I agree what our guys posted here about your question .

Its practical to just configure it point to point for /30 for areas not bigger that about 10 routers.

DR/BDR election will be necessary for populated areas to reduce LSA exchanges of each and every router. Its just like a client-server stuff.


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