When is DR and BDR required?

[jacobholmjensen]

Hi all, 

I'm currently, somehow, understanding how OSPF works and how elections and network advertising takes place. However, I feel like I'm missing a very important, and basic, point of when DR/BDR is necessary.

From what I can read, but not understand, the DR/BDR election is required when routers are interconnected over a common Ethernet network. But how come? 

In this topology there is no need for DR/BDR.

But in this one there is:

It might be my overthinking of what an Ethernet network is or what a switch does that's my worst enemy, but I can't seem to make sense/logic of it.

I hope some of you can dumb it down for me

Best,

Jacob 

[Jon Marshall · Accepted solution]

It is because there "could" be multiple routers on a shared network segment that OSPF elects a DR/BDR not that there are actually multiple routers. 

Jon

[jacobholmjensen]

Jon, you truly are a community legend. I can't believe that all it took for you was two lines.

Thank you doesn't start to cover my appreciation, but... Thank you very, very much!

[Jon Marshall]

No problem at all. 

Jon

[Flavio Miranda]

Hi

  You actually understood correctly and for most of scenario, DR and BDR is not necessary.  The requirement for DR and BDR is actually the medium. If you connect the router to a Broadcast network like ethernet there will be DR and BDR and if you connect the routers using a non-broadcast network like Serial, there will be no election. This is more link a prevention algothm since that  in broadcast network you have the "possibility" of many routers seeing each others and exchange LSA, but not necessary this will happen. You can have two routers connected using Ethernet.

  But is it important to mention that OSPF LSA are sent to a multicast IP address so, if you have too much routers in a network using broadcast medium like Ethernet, you can have performance problem.

  Long story short, this is more like a precaution than a necessity.

[David Ruess]

Hello,

By default depending on the media used OSPF will automatically select the type of link and therefore determining weather or not a DR/BDR is needed. Typically you need a DR/BDR if MORE than 2 routers occupy a segment (on same IP space and connected in a multiaccess network).

Sometimes even a point to point link will elect a DR/BDR based on the type of link connecting them since its auto-negotiated basically. In your case the serial links wont elect a DR/BDR because of the link types.

As @Jon Marshall pointed out there "could" be multiple routers on the LAN segment of the 2nd picture. So even if you only had 2 routers connected to the switch, the fact that they are on an ethernet LAN segment the link type has determined that a DR/BDR needs to be elected.

You can also influence which devices become the DR/BDR with manual configurations as well as the network type to prevent or enable DR.BDR elections. The DR/BDR is helpful to multiaccess segments because there is a central location the routers send their LSAs to.

Hope that helps

-David

[Joseph W. Doherty]

Just to touch (possibly again - i.e. mentioned by other posters) on some points about DR/BDR, it happens, by default for OSPF routers when they are on what OSPF considers to be shared access media, such as Ethernet.

In your OP, although your first diagram shows p2p networks, between the 3 routers, if your "WAN" links had Ethernet interfaces, each of those p2p links would elect one router as DR and one as BDR, for each network segment.  (I.e. you would have 3 DRs and 3 BDRs.)

In your OP's second diagram, all three routers are on a shared (Ethernet?) media, so again, DR/BDR would be elected, even if there were only 2 routers, rather than 3.

Of course, you probably realize the whole point of DR/BDR is to avoid the need for a "full mesh" of OSPF routers, directly sharing OSPF info, which, BTW, you still have with 2 or 3 routers.

When you do have p2p on Ethernet, and just a pair of OSPF routers, it's often good practice to configure OSPF, on that network, as p2p to avoid creating DR/BDR.