Is It Necessary to Specify DR and BDR manually?

[goldpartner]

Is it necessary to specify DR and BDR manually from the perspective of best OSPF design practice?

[David Ruess]

Hello,

In general it is better to specify how you want the network to behave rather than let the network decide by defaults. This is true for any protocol. If you just have 2 routers connecting to each other it doesn't matter who the DR/BDR is because the result is the same, but even then you should change the Network type to Point-to-Point to reduce LSA flooding and propagation therefore reducing link utilization. In certain HUB/Spoke designs such as the case with DMVPN it is needed to control the DR/BDR election because if a Spoke gets elected DR then you will have routing issues and will not have full connectivity. There are many reasons to and not to manually control it. The important thing is you know that it can be done and how its achieved and what the end result is.

Hope that helps

-David

[M02@rt37]

Hello @goldpartner,

You can influence the DR and BDR election process by manually configuring the OSPF priority. Use the 

ip ospf priority X

 command under

interface

command.

In some cases, you might want to exert control over the DR and BDR elections to ensure that specific routers take on these roles based on your network design, redundancy requirements, or performance considerations. 

[goldpartner]

Thanks for your answer. Is it possible to provide more details about the scenarios when considering redundancy requirements or performance?

[David Ruess]

The examples I provided above highlight both of these considerations. P2P links and DMVPN are prime examples where you want to control the DR/BDR or eliminate the election all together.

[M02@rt37 · Accepted solution]

You're welcome @goldpartner.

In a Frame Relay network, where multipoint subinterfaces are used, controlling DR and BDR elections is essential to ensure optimal traffic routing. Assigning the central hub router as the DR helps manage traffic efficiently across the Frame Relay network.

In DMVPN, net admin often assign the hub as the DR to ensure that all traffic flows through the hub, facilitating optimal communication between the spokes.

On the other hand, the absence of DR and BDR elections in point-to-point networks simplifies OSPF operations and reduces the overhead associated with managing DR and BDR roles. This results in efficient and straightforward OSPF neighbor relationships, making it suitable for point-to-point links where only two devices are involved.

[David Ruess]

M02@rt37 and @goldpartner I would make a some changes in this answer below to prevent confusion.

---

M02@rt37 wrote:

In DMVPN, net admin often assign the hub as the DR to ensure that all traffic flows through the hub, facilitating optimal communication between the spokes.

---

You say the net admin often assigns the HUB as DR, I would say required to configure the HUB as DR. If any spoke is elected the DR the DMVPN network would not function and routing would be broken. The only exception being a network with1 HUB and 1 Spoke, but at that point its a P2P and there is no need for a DR/BDR.

Side note: Th reason for this is the DR sends the Type 2 LSA to all the spokes for the network information of the broadcast link. If a Spoke was the DR it could only send the Type 2 LSA to the HUB as it wont have routing relationships with the other spoke routers to get this information to them so routing would be broken.

Secondly you mention its so ALL traffic flows through the HUB. The Spoke NHRP messages Requests the NBMA address of the remote Spoke form the HUB but it forwards traffic to the spoke directly as the DR does not modify the Next Hop of the route unlike EIGRP in certain situations. Traffic doesn't flow through the DR/HUB in a broadcast network on DMVPN (that defeats the purpose of Spoke to Spoke communication) unless the network is through the DR itself. This is also the case in a regular OSPF broadcast network without DMVPN. Traffic is sent to the spoke never reaching the DR. 

Hope explanations helped and made sense as I struggled with DMVPN for a long time trying to understand its workings.

[MHM Cisco World]

https://journey2theccie.wordpress.com/2020/04/21/dmvpn-phase-3-ospf/

Only case I see it neccessary is for dmvpn ospf broadcast network type 

Check link

[Joseph W. Doherty]

---

@goldpartner wrote:

Is it necessary to specify DR and BDR manually from the perspective of best OSPF design practice?

---

I would say NO, because DR is not preemptive.  I.e. You cannot guarantee which router will be the DR when a BDR is involved.

On the other hand, I would say whether you should be using DR/BDRs or use a DR with other routers set so them cannot become a BDR (or DR), would be part of best OSPF design practice.