DR and BDR in OSPF

[ranjipremandrewp]

Let us assume that there are two routers.

R1 has one physical interface with IP address 192.168.1.2 (Router ID will be 192.168.1.2)

R2 has one physical interface with IP address 192.168.1.1 and a logical interface with IP address 10.0.0.1 (Router ID will be 10.0.0.1)

Question: Who will become the DR for this segment? R1 or R2 and why?

[chelu.marius931 · Accepted solution]

In the election of the designated and backup router the DR is the one with the highest priority or the highest router id.Assuming you let the priority at the default 1,R1 will be the DR because it has the highest router id.

[Mark Malone · Accepted solution]

It chooses in the following order

1- Choose the highest OSPF priority (default is 1, note that if you configure priority of 0 that router wont enter the election process )

2- Choose the highest router-id

3- Choose the highest loopback interface

4- Choose the highest configured physical interface (must be up/up)

[InayathUlla Sharieff]

 
OSPF election process happens in the Init phase during wich OSPF Hello protocol (multicast) is in charge of the DR/BDR elections for a network segment combining  various factors in the following order:
 
1- Choose the highest OSPF priority (default is 1, note that if you configure priority of 0 that router wont enter the election process - used on FR networks.)
2- Choose the highest router-id
3- Choose the highest loopback interface
4- Choose the highest configured physical interface (must be up/up)
 
Mind that there is no preemption meaning that when a DR goes ofline, the BDR takes his place and when it comes back online it wont be the DR again, for it to be considered for another election process in a network segment, all DR/BDR must go offline so that one router can become the DR again for that segment.

Next:----

There are three steps of DR & BDR Election :
 
 
step 1 . A router starting the ospf process and listen for ospf hellos, if none are heard with in the dead time, it declare itself the DR.
 
step 2 . If hellos from other routers are heard , the router with the highest ospf priority is elected DR, and election process start again for BDR. A priority of zero removes a router from the election.
 
step 3. If two or more routers have the same ospf priority , the router with the highest router-id is elected DR, and the election process start again for BDR. After a DR is elected , election do not take place again unless the DR or BDR are lost. default priority is 1.
 
 

Hth

Regards

Inayath

[ranjipremandrewp]

Hello Inayath,

Thank you very much for the detailed explanation.

Ranji