OSPF DR/BDR Election explained

OSPF uses a DR (Designated Router) and BDR (Backup Designated Router) on each multi-access network to reduce routing update traffic and LSA flooding. If you have multiple routers on a multi-access segment, and all those routers would form neighbor adjacencies with each other, you would have a lot of unnecessary LSA flooding. The DR acts as a central point for the distribution of LSAs.

This election happens per multi-access segment, not per area. A multi-access network is a network segment with more than two routers where routers can reach each other directly. OSPF figures out whether a network is multi-access or not by looking at the interface type. For example, an Ethernet interface is considered multi-access, and a serial interface with PPP encapsulation is considered a point-to-point interface. OSPF automatically sets the correct OSPF network type for each interface.

Key Takeaways

• DR/BDR election occurs per multi-access segment, not per OSPF area
• Default OSPF priority is 1. The higher priority wins the election
• Setting priority to 0 prevents a router from becoming DR or BDR
• Election requires clear ip ospf process command to take effect after priority changes
• Point-to-point links (like serial interfaces) don’t use DR/BDR
• Routers that are neither DR nor BDR are called DROTHER

Configuration

Let’s look at this in action.

Multi-Access Network

We’ll start with an example of a multi-access network with a DR/BDR election. 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’s 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. 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.

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

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

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

In the example above, we have two multi-access segments between R2 and R1, and between R2 and R3. For each segment, there will be a DR/BDR election.