DR/BDR election in OSPF


(Naresh N) #1

Hi Rene,

You said the DR/BDR are not limited to an area in one of the chapters, I have not yet found an answer to this. Could you please point me to a link that describes why this is so and if new areas are added, how DR/ BDR may be impacted? BTW I liked how you have explained concepts in simple and elegant manner.

Thanks
Naresh


(krishnakumar T) #2

DR Election process is not preemptive


(Lazaros Agapides) #3

Hello Naresh

Rene mentions that the DR/BDR are not elected on a per area basis. Actually, there can be many DR/BDRs within a single area. The DR/BDR are elected on a per broadcast domain basis. This can be more clearly seen when we define what their purpose is.

If you have multiple routers within the same subnet/VLAN/broadcast domain, then you need a DR/BDR election. For example, take a look at this diagram:
image

There are two broadcast domains in this area 0: the one between R1 and R2 and the one between R2 and R3. Even through there are two routers in each broadcast domain, a DR/BDR election still takes place because the technology used is Ethernet, which is multi access. Unless you configure it otherwise, an election will still take place. You can imagine that more routers are connected to each switch if you like. So an election will take place between R1 and R2 for that broadcast domain, and let’s say R1 becomes the DR for that. Similarly, another election takes place between R2 and R3. Let’s say that R2 becomes the DR for that segment.

So you can see that a single router may be DR for a particular segment, and BDR (or DROTHER) for another segment, just like R2 is DR for the R2-R3 segment while it is BDR for the R1-R2 segment.

You can find out more about DR/BDR elections at this lesson, where Rene mentions the fact that DR/BDRs are elected on a per segment basis.

I hope this has been helpful!

Laz