OSPF DR and BDR

This lesson demonstrates the OSPF concept of a Designated Router (DR) and a Backup Designated router (BDR). We discuss why we need DR and BDR in the first place. Then, we walk through the process of electing DR and BDR on a multiaccess segment and show some advanced examples.

Since R1 is a brand-new router that has just been connected to the OSPF domain, its link-state database is basically empty. When R1 becomes a 2-way neighbor with each router, it then transitions to Exstart/Exchange/Loading phases and exchanges link-state information with every router. However, since all seven routers, R2 through R8, are in the same OSPF Area, they have identical link-state databases (LSDB). So, what actually happens is that R1 receives the same LSDB database seven times in a row. Does this seem efficient? Obviously not.

In the past, when routers had only a few MB of RAM and a single slow CPU, this inefficiency was a big deal. (the protocol is 30+ years old) That’s why network architects started to think of ways to optimize this process and make it work efficiently on multiaccess networks such as Ethernet LANs.

Let’s think about the inefficiency for a while:

R1 is a brand-new device with an empty LSDB database.
R2 through R8 all have identical LSDBs.

Logically, it is enough for R1 to only receive the LSDB from one of the neighbors since all have identical LSDBs. But from which one? Well, this is the concept of the Designated Router – an automatically elected single router that handles the LSDB exchange on the shared LAN. Every other router exchanges link-state information only with the DR instead of separately with all neighbors.

The LSA flooding on shared LANs

Additionally, every router becomes fully adjacent (neighbor in the state: Full) to every other device in the topology. This means there are n(n-1)/2 adjacencies on the segment, as shown in the diagram below. This is also inefficient.

Let’s compare this scenario with the one we showed earlier in Figure 4:

Without a Designated Router: With a full mesh of 23 OSPF adjacencies in a full state, 23 different instances of a database exchange will occur. Every router exchanges LSDB with seven others.
With a Designated Router: With the introduction of a designated router (DR), every router performs a full database exchange ONLY with the DR.

This is a massive optimization of the shared LAN segment. Especially back in the old days when routers had a few MB of RAM and a single slow CPU.

How does OSPF DR and BDR work?

Now, let’s zoom in a bit and see how the DR/BDR election process works.

The Election Process

The DR election process is based on a parameter in the OSPF Hello packet called Priority, which has values from 0 to 255 (2⁸).

By default, every router has Priority 1.
A router with a higher priority value is eligible to be elected as the Designated Router (DR) on the VLAN segment.
A router with priority 0 is ignored in the election process.
If priorities are equal, the highest Router ID breaks the tie.

It is very important to remember the following aspects of the election process from the very beginning:

Each router performs the DR/BDR election process locally with the information collected from neighbors’ Hello packets. However, every device’s algorithm is the same, so everyone reaches the same result.
There is no preemption in the DR/BDR process! Once a device is elected the DR, another device cannot preempt the role until the current DR’s OSPF process restarts. Even if a higher-priority device connects to the LAN, it cannot become DR until the current DR fails.

Let’s walk through a couple of scenarios to demonstrate how the election process works.

Scenario 1: DR election on a new link

We have a simple topology of four devices connected to the same VLAN via a layer two switch that has just been powered on. This clarification is very important because it means no DR has been elected yet, and the process starts from scratch.

By default, every router sends a Hello packet every 10 seconds. In the Hello packet, every router includes its RID, the RIDs of other neighbors it hears, the default Priority of 1, and an empty DR and BDR IP address. Notice that the DR and BDR IP of 0.0.0.0 indicates that no Designated or Backup Designated routers have been elected yet.

During the WAIT interval of 40 seconds (equal to the DEAD timer or four Hello intervals), none of the routers can claim themselves as DR or BDR. Everybody is just listening. Additionally, none of the routers transition to Exstart/Exchange/Loading/Full neighbor states. Everyone is waiting for the DR/BDR election process to finish first.

The following diagram shows the OSPF neighboring states that routers go through before becoming fully adjacent and exchanging their LSDB. On multiaccess segments such as an Ethernet VLAN, routers become fully adjacent only with the DR and the BDR when such are elected.

				
					R1# sh ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   2WAY/DROTHER    00:00:38    10.1.1.2        Ethernet0/0
3.3.3.3           1   2WAY/DROTHER    00:00:39    10.1.1.3        Ethernet0/0
4.4.4.4           1   2WAY/DROTHER    00:00:38    10.1.1.4        Ethernet0/0

				
					R1# sh ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   2WAY/DROTHER    00:00:39    10.1.1.2        Ethernet0/0
3.3.3.3           1   FULL/BDR        00:00:32    10.1.1.3        Ethernet0/0
4.4.4.4           1   FULL/DR         00:00:31    10.1.1.4        Ethernet0/0

				
					R1# sh ip ospf interface e0/0
Ethernet0/0 is up, line protocol is up 
  Internet Address 10.1.1.1/24, Interface ID 2, Area 0
  Attached via Network Statement
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 10
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           10        no          no            Base
  Transmit Delay is 1 sec, State DROTHER, Priority 1,
  Designated Router (ID) 4.4.4.4, Interface address 10.1.1.4
  Backup Designated router (ID) 3.3.3.3, Interface address 10.1.1.3
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:07
  Supports Link-local Signaling (LLS)
  Cisco NSF helper support enabled
  IETF NSF helper support enabled
  Can be protected by per-prefix Loop-Free FastReroute
  Can be used for per-prefix Loop-Free FastReroute repair paths
  Not Protected by per-prefix TI-LFA
  Index 1/1/1, flood queue length 0
  Next 0x0(0)/0x0(0)/0x0(0)
  Last flood scan length is 0, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 3, Adjacent neighbor count is 2 
    Adjacent with neighbor 3.3.3.3  (Backup Designated Router)
    Adjacent with neighbor 4.4.4.4  (Designated Router)
  Suppress hello for 0 neighbor(s)

				
					R1# sh ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   2WAY/DROTHER    00:00:30    10.1.1.2        Ethernet0/0
3.3.3.3           1   FULL/BDR        00:00:32    10.1.1.3        Ethernet0/0
4.4.4.4           1   FULL/DR         00:00:32    10.1.1.4        Ethernet0/0

R1 has just been connected to the VLAN. It is configured with an OSPF Priority of 50. Let’s see what happens when it connects to the segment and hears the Hello messages of the other OSPF devices.

When R1’s interface that connects to the VLAN comes up, it enters the INIT state. The OSPF WAIT timer starts. The device starts listening for OSPF Hello packets from other OSPF-enabled devices on the LAN.

If, during the duration of the WAIT timer, the router receives a Hello packet with the IP address of DR and BDR, the router immediately proceeds to the DR/BDR election process without waiting for the WAI timer to expire. Otherwise, the router waits for the full WAIT interval to expire.

In our example, when R1 hears the Hellos from the other routers, it immediately understands that R4 is currently the elected DR and R3 is the BDR. Although R1 has higher OSPF priority than the current DR/BDR, R1 cannot preempt the existing DR and take over the role. Only if R4’s OSPF process restarts can R1 become a Designated Router.

We can verify this by checking the OSPF neighbor table of R1.

				
					R1# sh ip ospf neighbor 
Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   2WAY/DROTHER    00:00:32    10.1.1.2        Ethernet0/0
3.3.3.3           1   FULL/BDR        00:00:35    10.1.1.3        Ethernet0/0
4.4.4.4           1   FULL/DR         00:00:32    10.1.1.4        Ethernet0/0

				
					R1# sh ip ospf interface e0/0
Ethernet0/0 is up, line protocol is up 
  Internet Address 10.1.1.1/24, Interface ID 2, Area 0
  Attached via Network Statement
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 10
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           10        no          no            Base
  Transmit Delay is 1 sec, State DROTHER, Priority 50
  Designated Router (ID) 4.4.4.4, Interface address 10.1.1.4
  Backup Designated router (ID) 3.3.3.3, Interface address 10.1.1.3
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:00
  Supports Link-local Signaling (LLS)
  Cisco NSF helper support enabled
  IETF NSF helper support enabled
  Can be protected by per-prefix Loop-Free FastReroute
  Can be used for per-prefix Loop-Free FastReroute repair paths
  Not Protected by per-prefix TI-LFA
  Index 1/1/1, flood queue length 0
  Next 0x0(0)/0x0(0)/0x0(0)
  Last flood scan length is 0, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 3, Adjacent neighbor count is 2 
    Adjacent with neighbor 3.3.3.3  (Backup Designated Router)
    Adjacent with neighbor 4.4.4.4  (Designated Router)
  Suppress hello for 0 neighbor(s)

				
					R4# clear ip ospf process 
Reset ALL OSPF processes? [no]: yes
R4#
*Jul 22 15:25:04.969: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on Ethernet0/0 from FULL to DOWN, Neighbor Down: Interface down or detached
*Jul 22 15:25:04.969: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on Ethernet0/0 from FULL to DOWN, Neighbor Down: Interface down or detached
*Jul 22 15:25:04.969: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on Ethernet0/0 from FULL to DOWN, Neighbor Down: Interface down or detached

				
					R1# sh ip ospf interface e0/0
Ethernet0/0 is up, line protocol is up 
  Internet Address 10.1.1.1/24, Interface ID 2, Area 0
  Attached via Network Statement
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 10
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           10        no          no            Base
  Transmit Delay is 1 sec, State BDR, Priority 50
  Designated Router (ID) 3.3.3.3, Interface address 10.1.1.3
  Backup Designated router (ID) 1.1.1.1, Interface address 10.1.1.1
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
. . .

				
					R3# clear ip ospf process 
Reset ALL OSPF processes? [no]: yes
R3#
*Jul 22 15:32:01.156: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on Ethernet0/0 from FULL to DOWN, Neighbor Down: Interface down or detached
*Jul 22 15:32:01.156: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on Ethernet0/0 from FULL to DOWN, Neighbor Down: Interface down or detached
*Jul 22 15:32:01.156: %OSPF-5-ADJCHG: Process 1, Nbr 4.4.4.4 on Ethernet0/0 from FULL to DOWN, Neighbor Down: Interface down or detached

				
					R1# sh ip ospf interface e0/0
Ethernet0/0 is up, line protocol is up 
  Internet Address 10.1.1.1/24, Interface ID 2, Area 0
  Attached via Network Statement
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 10
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           10        no          no            Base
  Transmit Delay is 1 sec, State DR, Priority 50
  Designated Router (ID) 1.1.1.1, Interface address 10.1.1.1
  Backup Designated router (ID) 4.4.4.4, Interface address 10.1.1.4
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:04
  Supports Link-local Signaling (LLS)
  Cisco NSF helper support enabled
  IETF NSF helper support enabled
  Can be protected by per-prefix Loop-Free FastReroute
  Can be used for per-prefix Loop-Free FastReroute repair paths
  Not Protected by per-prefix TI-LFA
  Index 1/1/1, flood queue length 0
  Next 0x0(0)/0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 3
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 3, Adjacent neighbor count is 3 
    Adjacent with neighbor 2.2.2.2
    Adjacent with neighbor 3.3.3.3
    Adjacent with neighbor 4.4.4.4  (Backup Designated Router)
  Suppress hello for 0 neighbor(s)

Notice which router has become the BDR—R4. That’s because the other three routers have a default priority of 1, and R4 has the highest Router ID, 4.4.4.4.

The LSA flooding process with the DR

Now, let’s quickly walk through the LSA flooding process on a multiaccess segment such as an Ethernet VLAN. Since on multiaccess segments OSPF uses Designated Routers, the LSA flooding process happens in two steps.

Step 1. DROTHER routers (non-DR/BDR routers) send the LSA updates to the link-local multicast IPv4 address 224.0.0.6. Only the Designated and Backup Designated routers listen to that multicast address. Hence, only the DR and the BDR receive the update. (technically, the packet goes to all devices in the LAN because link-local multicast is treated as a broadcast in the LAN, but only the DR and BDR process it). For example, when R5 sends an LSA update to 224.0.0.6, only the DR and BDR get the update, as shown in the diagram below.

You can see that the LSA flooding process works in a hub-and-spoke manner, with the DR acting as the hub. The hub-and-spoke LSA flooding behavior is achieved using the two different IPv4 link-local multicast addresses.

DROther routers send LSRs/LSUs/LSAcks to 224.0.0.6.
DR/BDR send LSRs/LSUs/LSAcks to 224.0.0.5.

Key Takeaways

OSPF uses the concept of Designated Routers (DR and BDR) on multiaccess segments such as an Ethernet VLAN.

Without a DR, every router would need to establish a full mesh of adjacencies with every other router on the same VLAN. In a network with n routers, this would require n(n-1)/2 full adjacencies, leading to significant overhead in LSDB exchange and SPF processing time.
With a DR, each router only forms an adjacency with the DR and Backup Designated Router (BDR). This reduces the number of required full-state adjacencies to 2(n-1), significantly lowering the control-plane overhead. The DR is responsible for flooding LSAs to all other routers on the segment. This prevents every router from flooding LSAs to every other, thereby reducing unnecessary traffic.
In the context of Designated Routers, there are three roles in the multiaccess segment, as shown in the diagram below:

By default, every router has Priority 1.
A router with a higher priority value is eligible to be elected as the Designated Router (DR) on the VLAN segment.
A router with priority 0 is ignored in the election process.
If priorities are equal, the highest Router ID breaks the tie.

It is very important to remember the following aspects of the election process from the very beginning:

Each router performs the DR/BDR election process locally with the information collected from neighbors’ Hello packets. However, every device’s algorithm is the same, so everyone reaches the same result.
There is no preemption in the DR/BDR process! Once a device is elected the DR, another device cannot preempt the role until the current DR’s OSPF process restarts. Even if a higher-priority device connects to the LAN, it cannot become DR until the current DR fails.
- When the multiaccess segment initializes for the first time, all routers start their WAIT timers. During that period, they only listened to and collected information about neighbors in the segment.
- When the WAIT interval passes, every router arrives at the same conclusion about who the DR and the BDR are.
- If a router with higher priority or higher RID connects to the LAN after that, it cannot preempt any of the roles – DR or BDR. When the DR fails, the BDR takes over. When both the DR and BDR fail, the new router can become the DR.

OSPF DR BDR

In Open Shortest Path First (OSPF), DR and BDR are special routers elected in a broadcast network (like Ethernet) to reduce routing traffic and make the network more efficient.

1. DR (Designated Router)

DR (Designated Router) is the main router responsible for exchanging routing information with all other routers in the OSPF network segment.

Functions of DR:

Collects LSA (Link State Advertisement) from all routers.
Creates and distributes the LSDB (Link State Database) to other routers.
Reduces the number of adjacencies between routers.
Acts as the central communication point in the OSPF network segment.

Example:
If 5 routers are connected in the same LAN, instead of each router forming adjacency with all others, they send updates to the DR, and the DR distributes them to the rest.

2. BDR (Backup Designated Router)

BDR (Backup Designated Router) is the backup router for the DR.

Functions of BDR:

Monitors the DR continuously.
If the DR fails, the BDR immediately becomes the new DR.
Ensures high availability and prevents network disruption.

3. Why DR and BDR are needed

Without DR/BDR, every router must create adjacency with every other router.

Example with 5 routers:

Without DR/BDR → 10 adjacencies
With DR/BDR → Only DR and BDR handle most communication

This reduces network traffic and CPU usage.

4. DR and BDR Election Criteria

Routers elect DR and BDR based on:

OSPF Priority (highest wins)
Router ID (highest wins if priority same)

Priority range: 0 – 255

0 = Router cannot become DR/BDR
255 = Highest priority

Example:

Router	Priority	Router ID	Result
R1	1	1.1.1.1	DROther
R2	100	2.2.2.2	DR
R3	50	3.3.3.3	BDR

5. OSPF Router Roles

DR – Main router
BDR – Backup router
DROther – All other routers

6. DR/BDR Election Commands (Cisco)

Example in Cisco Systems router:

				
					interface g0/0
ip ospf priority 200
show ip ospf neighbor

Why Need DR BDR in OSPF

In Open Shortest Path First (OSPF), DR (Designated Router) and BDR (Backup Designated Router) are used to reduce routing traffic and make the network more efficient in broadcast networks like Ethernet.

1. Reduce Too Many Neighbor Connections

If many routers are in the same LAN, each router would normally connect (form adjacency) with every other router.

Example with 5 routers:

Without DR/BDR

R1 ↔ R2
R1 ↔ R3
R1 ↔ R4
R1 ↔ R5
R2 ↔ R3
R2 ↔ R4
R2 ↔ R5
R3 ↔ R4
R3 ↔ R5
R4 ↔ R5

Total 10 adjacencies

With DR/BDR

All routers connect mainly with DR and BDR

Total much fewer adjacencies

👉 This reduces network complexity.

2. Reduce LSA Flooding

Routers send LSA (Link State Advertisement) updates.

Without DR:

Every router sends updates to every router → too much traffic

With DR:

Routers send updates to DR
DR distributes updates to other routers

👉 Less routing traffic

3. Reduce CPU and Memory Load

If every router communicates with all routers:

CPU usage increases
Memory usage increases

DR/BDR centralizes communication, so routers use less CPU and resources.

4. Network Stability

BDR acts as backup.

If DR fails:

BDR immediately becomes new DR
Network continues working

👉 High availability

5. Faster Convergence

Because updates are controlled by DR:

Routing table updates become faster
Network becomes more stable

Short Answer:
DR and BDR are used in OSPF to reduce routing traffic, reduce adjacencies, improve network performance, and provide backup if the main router fails.

OSPF Router Roles

DR – Main router
BDR – Backup router
DROther – All other routers

6. DR/BDR Election Commands (Cisco)

Example in Cisco Systems router:

				
					interface g0/0
ip ospf priority 200
show ip ospf neighbor

1. Introduction

In OSPF (Open Shortest Path First), DR (Designated Router) and BDR (Backup Designated Router) are used to manage communication between multiple routers in a network. They are mainly required in multi-access networks like Ethernet LANs.

2. Problem Without DR/BDR

If DR and BDR are not used, every router must form a neighbor relationship with every other router. This creates a full mesh topology.

For example, if 5 routers are connected:

Each router connects with 4 others
Total adjacency = 10 connections

This leads to:

High CPU usage
High memory usage
Excessive LSA (Link-State Advertisement) traffic

3. Need for DR/BDR

To solve this problem, OSPF introduces DR and BDR. Instead of full mesh:

One router becomes DR
One router becomes BDR
All other routers connect only to DR/BDR

This reduces:

Number of adjacencies
Network traffic
Resource usage

4. Role of DR

The DR acts as a central communication point. All routers send their LSAs to the DR, and the DR distributes them to other routers. This avoids unnecessary duplication of messages.

5. Role of BDR

The BDR works as a backup router. It listens to all OSPF updates and stays ready. If the DR fails, the BDR immediately takes over as DR, ensuring no network interruption.

6. Example Scenario

Consider 4 routers in a LAN:

Router	Priority	Role
R1	100	DR
R2	90	BDR
R3	1	DROTHER
R4	1	DROTHER

Working:

R3 and R4 send updates to DR (R1)
DR sends updates to all routers
If R1 fails → R2 becomes DR automatically

7. Adjacency Comparison

Without DR/BDR:

4 routers → 6 adjacencies

With DR/BDR:

Only 3–4 adjacencies

This significantly improves performance.

8. Where DR/BDR is Used

Used in:

Broadcast networks (Ethernet)
Non-broadcast networks (NBMA)

Not used in:

Point-to-point links

9. DR/BDR Election Rule

Election is based on:

Highest OSPF Priority
If same → Highest Router ID

👉 Priority 0 = cannot become DR/BDR

10. Benefits

Reduces network complexity
Improves scalability
Minimizes LSA flooding
Provides backup (BDR)

Final Conclusion

DR and BDR are essential in OSPF to make the network efficient and scalable.

DR = Main communication router
BDR = Backup for reliability

Without DR/BDR, OSPF networks would become slow and overloaded in multi-access environments.

DR BDR Multicast IP Addresses

In Open Shortest Path First (OSPF), routers use multicast IP addresses to communicate with DR (Designated Router) and BDR (Backup Designated Router). These addresses help send routing updates efficiently in a network.

1. 224.0.0.5 – All SPF Routers Address

224.0.0.5 is the multicast address used by all OSPF routers.

Explanation:

Every OSPF router joins this multicast group.
Hello packets and some routing updates are sent to this address.
All routers on the network segment receive these packets.

Example:
Router R1 sends a Hello packet to 224.0.0.5 →
R2, R3, R4 (all OSPF routers) receive it.

2. 224.0.0.6 – All DR BDR Routers Address

224.0.0.6 is the multicast address used by DR and BDR routers.

Explanation:

Only DR and BDR listen to this address.
Other routers (called DROther) send their LSA updates to this address.
DR then distributes the information to all routers.

Example:
R1 (DROther) sends LSA → 224.0.0.6
DR receives it → DR sends update to 224.0.0.5 so all routers get it.

3. Communication Flow Example

Step-1: All routers send Hello → 224.0.0.5
Step-2: Routers elect DR and BDR
Step-3: DROther routers send updates → 224.0.0.6
Step-4: DR distributes updates → 224.0.0.5

4. Summary Table

Multicast Address	Name	Used For
224.0.0.5	AllSPFRouters	Communication between all OSPF routers
224.0.0.6	AllDRouters	Communication with DR and BDR

Short definition:

224.0.0.5 → All OSPF routers receive packets.
224.0.0.6 → Only DR and BDR receive packets.

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OSPF DR and BDR

OSPF DR BDR

1. DR (Designated Router)

2. BDR (Backup Designated Router)

3. Why DR and BDR are needed

4. DR and BDR Election Criteria

5. OSPF Router Roles

6. DR/BDR Election Commands (Cisco)

Why Need DR BDR in OSPF

1. Reduce Too Many Neighbor Connections

2. Reduce LSA Flooding

3. Reduce CPU and Memory Load

4. Network Stability

5. Faster Convergence

OSPF Router Roles

6. DR/BDR Election Commands (Cisco)

1. Introduction

2. Problem Without DR/BDR

3. Need for DR/BDR

4. Role of DR

5. Role of BDR

6. Example Scenario

7. Adjacency Comparison

8. Where DR/BDR is Used

9. DR/BDR Election Rule

10. Benefits

Final Conclusion

DR BDR Multicast IP Addresses

1. 224.0.0.5 – All SPF Routers Address

2. 224.0.0.6 – All DR BDR Routers Address

3. Communication Flow Example

4. Summary Table