On a multiaccess network like Ethernet, OSPF must reduce the number of adjacencies and the volume of Link State Advertisements (LSAs) flooding the segment. The Designated Router (DR) and Backup Designated Router (BDR) election solves this by centralizing LSA exchange through a single point. This topic is critical for the CCNA 200-301 exam (Objective 3.4) and for real-world OSPF design, as misconfigured priorities can lead to suboptimal traffic flow and troubleshooting nightmares.
Jump to a section
Imagine a small town with 30 houses, each needing to share announcements with every other house. If every house shouted its news to every other house simultaneously, the noise would be unbearable and many messages would be lost. So the town decides to elect a Mayor (DR) and a Deputy Mayor (BDR). Every house sends its announcements only to the Mayor, who then broadcasts them to all other houses. The Deputy Mayor listens to all announcements but stays silent unless the Mayor suddenly resigns—in which case the Deputy instantly becomes Mayor and a new Deputy is elected. The election happens when the town first forms or when a new house arrives. The Mayor is chosen based on seniority (Router ID) and willingness to serve (OSPF priority). If the Mayor fails, the Deputy takes over seamlessly, ensuring no announcements are lost. This system scales perfectly: with 30 houses, without a Mayor, you'd need 435 direct shouting pairs; with a Mayor, only 29 pairs (each house to Mayor, Mayor to all). The analogy teaches that the DR/BDR election reduces OSPF adjacencies from N*(N-1)/2 to 2*(N-1) on a broadcast multiaccess segment, and that the BDR provides fast failover without a new election storm.
Why OSPF Needs DR and BDR
On broadcast multiaccess networks (e.g., Ethernet), OSPF routers must form full adjacencies and exchange LSAs. Without a DR, every router would form a full adjacency with every other router, resulting in N*(N-1)/2 adjacencies. For N=10, that's 45 adjacencies. With a DR, each router forms a full adjacency only with the DR and BDR, yielding 2*(N-1) adjacencies (18 for N=10). The DR acts as the central point for LSA flooding: all other routers (DRothers) send their LSAs to the DR (and BDR) via multicast 224.0.0.6 (AllDRouters), and the DR floods them to all routers via multicast 224.0.0.5 (AllSPFRouters). The BDR listens to all traffic but does not flood; it takes over if the DR fails.
Election Process Step by Step
The election occurs on broadcast and NBMA networks when the interface state transitions to Waiting. The Waiting state lasts for the Dead Interval (default 40 seconds on broadcast). During this time, the router listens for Hello packets from potential DR/BDR. The election uses two criteria: highest OSPF priority (0-255, default 1) and, if priorities tie, highest Router ID. A priority of 0 means the router is ineligible to become DR or BDR. The election is non-preemptive: once a DR and BDR are elected, they remain until they fail, even if a higher-priority router joins later. A new election only occurs when the DR or BDR fails, or when the interface goes down.
Hello Packet Details
Hello packets are sent every 10 seconds (default Hello Interval) to multicast 224.0.0.5. They contain the router's Router ID, priority, Hello/Dead intervals, list of neighbors, and the DR and BDR IP addresses (0.0.0.0 if none elected). Routers use the DR and BDR fields in received Hellos to determine the current election state. The Dead Interval is 40 seconds by default. If no Hello is received from a neighbor within the Dead Interval, the neighbor is declared dead, which may trigger a new election.
OSPF Interface States Related to Election
Down: Initial state, no Hellos sent.
Attempt: Only on NBMA networks; sending Hellos to neighbors manually configured.
Init: Hello received but not seeing own Router ID in neighbor's Hello.
2-Way: Bidirectional communication established; both routers see each other in Hello packets. At this point, DR/BDR election takes place.
ExStart: Master/slave negotiation for Database Description (DD) packets.
Exchange: DD packets exchanged to describe LSDB.
Loading: Link State Requests and Updates exchanged.
Full: Fully adjacent; routers have identical LSDBs.
Only routers in Full state with the DR/BDR are considered adjacent. DRothers never become Full with each other; they stay at 2-Way.
Verification Commands
Use the following IOS commands to verify DR/BDR status:
show ip ospf interface [interface-type number]
show ip ospf neighbor
show ip ospfExample output for show ip ospf interface GigabitEthernet0/0:
GigabitEthernet0/0 is up, line protocol is up
Internet Address 10.0.0.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State BDR, Priority 1
Designated Router (ID) 2.2.2.2, Interface address 10.0.0.2
Backup Designated Router (ID) 1.1.1.1, Interface address 10.0.0.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:03
Neighbor Count is 2, Adjacent neighbor count is 2
Adjacent with neighbor 2.2.2.2 (Designated Router)
Adjacent with neighbor 3.3.3.3
Suppress hello for 0 neighbor(s)Notice the "State BDR" line, and the DR/BDR IP addresses. The neighbor count is 2, but both are adjacent because one is DR and the other is a DRother that forms adjacency with BDR as well.
Interaction with Other OSPF Features
The DR/BDR election is independent of the OSPF area design and does not affect inter-area routing. However, on NBMA networks (Frame Relay, ATM), the election must be carefully managed because the network may not support broadcast. In such cases, the ip ospf network command can change the network type to point-to-multipoint or non-broadcast, which affects the election. Also, virtual links do not use DR/BDR.
Defaults and Timers
Hello Interval: 10 seconds (broadcast), 30 seconds (NBMA)
Dead Interval: 40 seconds (broadcast), 120 seconds (NBMA)
Wait Timer: equal to Dead Interval; determines how long the router waits before starting election
Priority: default 1; range 0-255
Router ID: highest loopback IP, else highest physical IP; can be manually set with router-id command
Set OSPF priorities per interface
Use the `ip ospf priority` command to influence the election. A higher priority makes a router more likely to become DR or BDR. A priority of 0 means the router will never become DR or BDR. For example, to set a router's GigabitEthernet0/0 priority to 100, enter interface configuration mode and issue: ``` interface GigabitEthernet0/0 ip ospf priority 100 ``` This command is executed before OSPF is enabled on the interface or before the election occurs. The priority is advertised in Hello packets. If you change priority after an election, it has no effect until the next election (triggered by DR/BDR failure).
Configure OSPF on interfaces
Enable OSPF on the interface with the `network` command under the OSPF process, or use the `ip ospf` command directly on the interface. The interface must be in an OSPF area. Example: ``` router ospf 1 network 10.0.0.0 0.0.0.255 area 0 ``` Alternatively, on the interface: ``` interface GigabitEthernet0/0 ip ospf 1 area 0 ``` Once OSPF is enabled, the interface begins sending Hello packets and the election process starts after the Wait timer expires (default 40 seconds).
Observe the election state machine
Use `debug ip ospf hello` and `debug ip ospf adj` to watch the election. Example debug output: ``` OSPF: 2 Way Comm with 2.2.2.2 on GigabitEthernet0/0 OSPF: Backup seen event before WAIT timer on GigabitEthernet0/0 OSPF: DR/BDR election on GigabitEthernet0/0 OSPF: Elect BDR 1.1.1.1 OSPF: Elect DR 2.2.2.2 OSPF: Wait timer expired on GigabitEthernet0/0 OSPF: Neighbor change event on GigabitEthernet0/0 ``` Notice the "2 Way Comm" indicates bidirectional communication. Then the election occurs. The Wait timer ensures enough time for all routers to send Hellos before deciding.
Verify the election results
After election, verify with `show ip ospf interface`. Look for "State" line (DR, BDR, or DROther) and the DR/BDR addresses. Example: ``` show ip ospf interface GigabitEthernet0/0 ``` Output snippet: ``` State DR, Priority 100 Designated Router (ID) 1.1.1.1, Interface address 10.0.0.1 Backup Designated Router (ID) 2.2.2.2, Interface address 10.0.0.2 ``` Also check `show ip ospf neighbor`: ``` show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 1 FULL/BDR 00:00:35 10.0.0.2 GigabitEthernet0/0 3.3.3.3 1 FULL/DROther 00:00:32 10.0.0.3 GigabitEthernet0/0 ``` The state column shows FULL/BDR or FULL/DROther, indicating adjacency type.
Force a new election by resetting DR
To trigger a new election, you can clear the OSPF process on the DR router (not recommended in production) or shut/no shut the interface on the DR. For example, on the DR router: ``` interface GigabitEthernet0/0 shutdown no shutdown ``` This causes the DR to go down, the BDR becomes DR, and a new BDR is elected from the remaining DROthers. Alternatively, you can clear the OSPF adjacency with `clear ip ospf process` (resets all adjacencies). Note: Changing priority does not trigger an election unless the DR/BDR fails.
Troubleshoot election issues
Common issues: mismatched Hello/Dead intervals prevent adjacency; priority 0 routers never become DR/BDR; Router ID tie-breaking misunderstood. Use `show ip ospf interface` to check intervals and priority. Use `show ip ospf neighbor` to see if neighbors are stuck in 2-Way (normal for DROthers) or not reaching Full. If a router with higher priority is not becoming DR, verify that the DR is still up; election is non-preemptive. Also check that the network type is broadcast (default on Ethernet). Use: ``` show ip ospf interface | include Network Type ``` If network type is point-to-point, no DR/BDR election occurs.
In enterprise networks, OSPF DR/BDR election is crucial for scalability. For example, in a campus LAN with 50 routers in a single VLAN, without DR, each router would form 49 adjacencies, resulting in 1225 total adjacencies—a huge burden on CPU and memory. With DR/BDR, each router forms only 2 adjacencies (to DR and BDR), reducing total to 98. This is particularly important on core switches where many distribution routers connect.
A common design practice is to set the OSPF priority on the most stable, high-performance router to a high value (e.g., 200) to ensure it becomes DR. The backup router gets priority 100, and all others remain at default 1. This avoids election surprises. However, if the DR is also the router performing inter-area summarization or redistribution, its CPU load increases; careful capacity planning is needed.
In production, a misconfiguration like setting priority 0 on all routers will result in no DR/BDR election, causing all routers to form full meshes—potentially crashing low-end routers. Another issue is using the same Router ID on two routers (e.g., both using the same loopback IP), which causes adjacency problems and election instability. Network engineers must ensure unique Router IDs, often by setting a loopback interface with a unique IP.
Performance considerations: On high-speed links (10 Gbps+), the DR can become a bottleneck if it handles all LSA flooding. In such cases, some designs use point-to-point links between routers instead of a multiaccess segment, eliminating the need for DR/BDR. Alternatively, OSPF can be configured in multiple areas to reduce LSA flooding scope.
When troubleshooting, if a router is not becoming DR despite high priority, check that the DR is still alive; non-preemption means you must reset the DR interface. Also verify that the network type is broadcast; if it's point-to-point, no election occurs.
The CCNA 200-301 exam tests OSPF DR/BDR election under Objective 3.4 (Configure and verify OSPF). Specifically, you must understand the election criteria (highest priority, then highest Router ID), the non-preemptive nature, and the fact that priority 0 means ineligible. You will see questions where a new router with higher priority joins an existing network; the correct answer is that it does NOT become DR because the election is non-preemptive. Many candidates incorrectly assume the new router takes over.
Another common trap: The exam may ask how many adjacencies are formed on a broadcast segment with N routers. The formula is 2*(N-1) (each router to DR and BDR, but DR and BDR also have adjacency to each other). However, some questions ask for the number of Full adjacencies; DROthers are only Full with DR and BDR, not with each other. So for 5 routers, there are 8 Full adjacencies (DR to 4 others, BDR to 4 others, but DR-BDR counted once? Actually each adjacency is counted once: DR has 4, BDR has 4, but DR-BDR is counted twice? No, adjacency is between two routers. So total = (N-1) for DR + (N-1) for BDR - 1 (DR-BDR counted twice) = 2*(N-1) - 1? Wait, careful: DR forms adjacency with N-1 routers (including BDR). BDR forms adjacency with N-1 routers (including DR). That's 2*(N-1) adjacencies, but the DR-BDR adjacency is counted twice. So total unique adjacencies = 2*(N-1) - 1. For N=5: 2*4-1=7. But many sources say 2*(N-1) = 8? Let's check: DR to 4, BDR to 4, but DR-BDR is one adjacency; so total 7. However, the CCNA often simplifies to 2*(N-1) because they consider DR and BDR as separate, but technically it's 2N-3. I've seen both. The safest is to know the concept: each DROther has 2 adjacencies (to DR and BDR), DR has N-1, BDR has N-1, and DR-BDR is one. So total = (N-1) + (N-1) - 1 = 2N-3. But the exam may expect 2*(N-1). Check the official curriculum: they say "each router forms a full adjacency with the DR and BDR." That implies each router has two adjacencies, so total adjacencies = 2*(N-1). But then the DR and BDR each have N-1 adjacencies, and the DR-BDR adjacency is counted twice. So the total number of adjacencies is N-1 (for DR) + N-1 (for BDR) - 1 (overlap) = 2N-3. I'll stick with 2N-3 for accuracy. However, the exam might not test the exact number; they might ask about the reduction.
Decision rule for scenario questions: If a router with priority 0 appears, it cannot be DR or BDR. If a new router joins, it does not trigger election unless DR fails. If DR fails, BDR becomes DR and a new BDR is elected (highest priority among remaining).
Also know the default timers: Hello 10, Dead 40, Wait 40. The Wait timer is the same as Dead timer and is used for election delay.
Finally, be able to interpret show ip ospf interface output, especially the State field and DR/BDR addresses.
DR/BDR election uses highest OSPF priority (0-255, default 1); tie broken by highest Router ID.
Priority 0 means the router is ineligible to become DR or BDR.
Election is non-preemptive: a higher-priority router joining later will not replace the current DR/BDR.
On broadcast multiaccess networks, each router forms full adjacencies only with the DR and BDR, reducing total adjacencies from N(N-1)/2 to 2N-3.
Hello interval defaults to 10 seconds on broadcast networks; Dead interval defaults to 40 seconds.
Use `show ip ospf interface` to verify DR/BDR status and `show ip ospf neighbor` to see adjacency states.
A new election is triggered only when the DR or BDR fails, or the interface goes down.
These come up on the exam all the time. Here's how to tell them apart.
OSPF DR/BDR Election
Elected on broadcast and NBMA networks.
Uses priority and Router ID.
Non-preemptive; election only at startup or failure.
DR and BDR centralize LSA flooding.
Reduces adjacencies from N(N-1)/2 to ~2N.
EIGRP Designated Router (No Election)
EIGRP does not use a DR concept; all routers form full adjacencies.
No election; every router peers with every other.
N/A; EIGRP uses Reliable Transport Protocol (RTP) for updates.
Each router sends updates to all neighbors using multicast 224.0.0.10.
Adjacencies are N(N-1)/2; no reduction.
Mistake
A router with a higher priority always becomes the DR immediately after joining.
Correct
The election is non-preemptive. Once a DR and BDR are elected, they remain until they fail, even if a higher-priority router joins later.
Candidates often assume that the election runs continuously, but it only happens at startup or after a failure.
Mistake
OSPF DR/BDR election occurs on point-to-point links.
Correct
DR/BDR election only occurs on broadcast and NBMA network types. On point-to-point links, no election happens; routers become fully adjacent directly.
Students confuse OSPF network types; point-to-point is common for serial links and does not use DR/BDR.
Mistake
The DR and BDR are elected based solely on Router ID.
Correct
The primary criterion is OSPF priority (higher is better). Only if priorities are equal does Router ID break the tie.
Many study materials emphasize Router ID, but priority is checked first. Candidates overlook the priority command.
Mistake
All routers on a broadcast segment must be fully adjacent to each other.
Correct
Only the DR and BDR form full adjacencies with every other router. DROthers remain in 2-Way state with each other and do not exchange LSAs directly.
The term 'adjacency' is often misused; candidates think all neighbors become fully adjacent, but OSPF limits full adjacencies to reduce LSA flooding.
Reveal each answer, then mark whether you got it right. Score 60%+ to unlock the next chapter.
The DR (Designated Router) is the central point for LSA flooding on a multiaccess network. It receives LSAs from all routers (via 224.0.0.6) and floods them to all routers (via 224.0.0.5). The BDR (Backup Designated Router) also receives all LSAs but does not flood them; it maintains a full adjacency with all routers to take over immediately if the DR fails. The BDR's role is to provide fast failover without a new election. Both are elected and remain until failure.
You cannot directly assign a router as DR, but you can influence the election by setting a high OSPF priority (using `ip ospf priority`) on the desired router. The router with the highest priority wins. If multiple routers tie, the highest Router ID breaks the tie. However, due to non-preemption, you must ensure the election happens after configuration (e.g., by reloading OSPF or shutting/no shutting interfaces).
When the DR fails, the BDR immediately takes over as the new DR. A new election is then held among the remaining routers (including the new DR) to select a new BDR. During this transition, OSPF continues to function because the BDR already has a full LSDB. The new BDR is elected based on priority and Router ID.
DROthers do not need to form full adjacencies because they exchange LSAs indirectly through the DR. Staying in 2-Way state is sufficient to maintain neighbor reachability. Full adjacency is only required with the DR and BDR to ensure LSAs are properly flooded. This reduces the number of adjacencies and LSA flooding overhead.
You can clear the OSPF process on the current DR using `clear ip ospf process`, which resets all adjacencies and triggers a new election. However, this disrupts routing. A less disruptive method is to change the priority on the DR to a lower value and then shut/no shut its interface, causing a new election. But both methods cause temporary outage. In production, it's better to plan the election during maintenance windows.
The Wait timer is equal to the Dead Interval (default 40 seconds on broadcast networks). It is used during the election process. When an OSPF interface comes up, it waits for the Wait timer to expire before starting the election, allowing time to receive Hello packets from all potential neighbors. This ensures that the election considers all routers on the segment.
No. On point-to-point links, OSPF forms a direct full adjacency between the two routers without any election. The network type is point-to-point, which does not support DR/BDR. Similarly, point-to-multipoint network types also do not use DR/BDR.
You've just covered OSPF DR and BDR Election — now see how well it sticks with free CCNA 200-301 practice questions. Full explanations included, no account needed.
Done with this chapter?