OSPF uses a DR (Designated Router) and BDR (Backup Designated Router) on each multi-access network. A multi-access network is a segment where we have more than two routers. OSPF figures this out by looking at the interface type. For example, an Ethernet interface is considered a multi-access network, and a serial interface is considered a point-to-point interface.
Most CCNA students think that this DR/BDR election is done per area but this is incorrect. I’ll show you how the election is done and how you can influence it. This is the topology we’ll use:
Here’s an example of a network with 3 OSPF routers on a FastEthernet network. They are connected to the same switch (multi-access network) so there will be a DR/BDR election. OSPF has been configured so all routers have become OSPF neighbors, let’s take a look:
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.123.2 1 FULL/BDR 00:00:32 192.168.123.2 FastEthernet0/0
192.168.123.3 1 FULL/DR 00:00:31 192.168.123.3 FastEthernet0/0
From R1 perspective, R2 is the BDR and R3 is the DR.
R3#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.123.1 1 FULL/DROTHER 00:00:36 192.168.123.1 FastEthernet0/0
192.168.123.2 1 FULL/BDR 00:00:39 192.168.123.2 FastEthernet0/0
When a router is not the DR or BDR it’s called a DROTHER. I have no idea if we have to pronounce it like “BROTHER with a D” or “DR-OTHER” 🙂 Here we can see that R1 is a DROTHER.
R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.123.1 1 FULL/DROTHER 00:00:31 192.168.123.1 FastEthernet0/0
192.168.123.3 1 FULL/DR 00:00:32 192.168.123.3 FastEthernet0/0
And R2 (the BDR) sees the DR and DROTHER.
Of course we can change which router becomes the DR/BDR by playing with the priority. Let’s turn R1 in the DR:
R1(config)#interface fastEthernet 0/0
R1(config-if)#ip ospf priority 200
You change the priority if you like by using the ip ospf priority command:
- The default priority is 1.
- A priority of 0 means you will never be elected as DR or BDR.
- You need to use clear ip ospf process before this change takes effect.
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.123.2 1 FULL/BDR 00:00:31 192.168.123.2 FastEthernet0/0
192.168.123.3 1 FULL/DR 00:00:32 192.168.123.3 FastEthernet0/0
As you can see R3 is still the DR, we need to reset the OSPF neighbor adjacencies so that we’ll elect the new DR and BDR.
R3#clear ip ospf process
Reset ALL OSPF processes? [no]: yes
R2#clear ip ospf process
Reset ALL OSPF processes? [no]: yes
I’ll reset all the OPSF neighbor adjacencies.
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.123.2 1 FULL/DROTHER 00:00:36 192.168.123.2 FastEthernet0/0
192.168.123.3 1 FULL/BDR 00:00:30 192.168.123.3 FastEthernet0/0
Now you can see R1 is the DR because the other routers are DROTHER and BDR.
R3#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.123.1 200 FULL/DR 00:00:30 192.168.123.1 FastEthernet0/0
192.168.123.2 1 FULL/DROTHER 00:00:31 192.168.123.2 FastEthernet0/0
Or we can confirm it from R3, you’ll see that R1 is the DR and that the priority is 200.
Configurations
Want to take a look for yourself? Here you will find the startup configuration of each device.
R1
hostname R1
!
ip cef
!
interface FastEthernet0/0
ip address 192.168.123.1 255.255.255.0
ip ospf priority 200
!
router ospf 1
network 192.168.123.0 0.0.0.255 area 0
!
end
R2
hostname R2
!
ip cef
!
interface FastEthernet0/0
ip address 192.168.123.2 255.255.255.0
!
router ospf 1
network 192.168.123.0 0.0.0.255 area 0
!
end
R3
hostname R3
!
ip cef
!
interface FastEthernet0/0
ip address 192.168.123.3 255.255.255.0
!
router ospf 1
network 192.168.123.0 0.0.0.255 area 0
!
end
Something you need to be aware of is that the DR/BDR election is per multi-access segment…not per area!). Let me give you an example:
In the example above we have 2 multi-access segments. Between R2 and R1, and between R2 and R3. For each segment, there will be a DR/BDR election.
Jason,
DRs and BDRs are not necessarily one-to-one with Areas. I think this is the key point to answer your question. DRs and BDRs are the result of the OSPF network type defined on a Router’s interface, not because of some requirement of an Area to have them. OSPF recognizes the following network types:
Out of all of those possibilities, only Broadcast and Non-Broadcast form DRs and BDRs. The Broadcast and Non-Broadcast network types describe
... Continue reading in our forumHi,
I have a question, In the process of election DR/ BDR, who will elect first? DR or BDR ?
I have only one router in my setup. I configured OSPF on that router and I started debug message. After the end of wait period ( dead interval 40 sec), router considers himself as BDR and then moved to DR .
please explain me, in the election process, will BDR is elected first or DR ?
Below is the debug message:
... Continue reading in our forumAzeem,
The BDR is elected first (see step #7 below). Here is a break down of what happens during the election:
David,
The BDR is elected first (see step 7 below). Here is a detailed list of what happens in the DR/BDR election:
Hello Americo
When we have a BMA situation, and all routers boot up together, ALL routers declare that they are the DR. So at the beginning, the subset of all routers not claiming to be the DR is an empty set. There are NO routers in this subset. So initially, this list of eligible routers will be empty. As hello packets are exchanged however, routers will receive the priority and router IDs and any routers ineligible to be a DR will CHANGE that information in the next hellos.
Now if a router creates a subset of all routers not claiming to be the DR, then
... Continue reading in our forum