VMware ESXi and vSphere Cluster Management
OSPF Designated Router (DR) and Backup Designated Router (BDR)
Learn how OSPF DR and BDR elections reduce adjacencies on multiaccess networks, how priorities and router IDs decide roles, and how to verify and troubleshoot them.
On a shared OSPF network, several routers may communicate through the same segment. OSPF uses a Designated Router (DR) and a Backup Designated Router (BDR) to reduce unnecessary adjacencies and control link-state flooding.
This lesson explains DR and BDR purpose, election rules, neighbor states, multicast behavior, failure recovery, Cisco IOS verification, and configuration techniques.
Why OSPF Uses a DR and BDR
A multiaccess network is a shared segment on which more than two OSPF routers can communicate. An Ethernet LAN is a common example.
Without a DR, every router on a shared segment would need a full adjacency with every other router. If a segment has n routers, the number of possible two-router relationships is:
n × (n - 1) / 2For four routers, that is six possible full adjacencies. OSPF instead uses a hub-like model. Each DROTHER forms a full adjacency with the DR and BDR, while two DROTHER routers normally remain in the 2-Way state with each other.
The DR centralizes important activity on the segment. It originates the Type 2 Network LSA that represents the shared transit network and floods topology changes to the OSPF routers on that segment.
The BDR is a fully synchronized standby. It maintains full adjacencies and keeps its link-state information current, but it does not perform the DR's normal network-LSA origination role while the DR is operating. If the DR fails, the BDR can take over quickly.
Network Types That Elect a DR and BDR
| OSPF network type | Typical medium | DR/BDR election | Neighbor behavior | Multicast availability |
|---|---|---|---|---|
| Broadcast | Ethernet LAN | Yes, by default | Full to DR and BDR; normally 2-Way between DROTHER routers | Native multicast is available |
| NBMA | Non-broadcast multiaccess technology | Yes | Uses DR/BDR relationships, but neighbor discovery and flooding depend on the NBMA configuration | No native broadcast or multicast behavior |
| Point-to-point | Two-router link | No | The two routers can form a direct full adjacency | Network behavior does not require DR/BDR |
| Point-to-multipoint | Hub-and-spoke or logical multipoint connection | No | Neighbors form direct relationships without a DR/BDR election | Depends on the underlying implementation |
Broadcast Ethernet interfaces normally use the broadcast OSPF network type. NBMA networks also use DR and BDR elections, but their lack of native multicast means that neighbor discovery and packet delivery require appropriate configuration. Point-to-point and point-to-multipoint networks do not elect a DR or BDR.
OSPF Roles and Adjacency States
OSPF routers discover one another by exchanging Hello packets. After discovery, they negotiate database synchronization and may form a full adjacency, meaning their link-state databases are synchronized.
A DROTHER is any router on the multiaccess segment that is neither the DR nor the BDR.
| Local role | Neighbor role | Expected state | Full adjacency? |
|---|---|---|---|
| DROTHER | DR | FULL/DR | Yes |
| DROTHER | BDR | FULL/BDR | Yes |
| DROTHER | DROTHER | 2WAY/DROTHER | Normally no |
| DR | BDR | FULL/BDR | Yes |
| DR or BDR | Eligible router | FULL/DR or FULL/BDR, as appropriate | Yes |
For example, if four routers share a broadcast LAN, one becomes DR, one becomes BDR, and the other two become DROTHER routers. Each DROTHER has full adjacencies with the DR and BDR. The two DROTHER routers normally show 2WAY with each other. This is expected behavior, not an adjacency failure.
DR and BDR Election Rules
The election uses the interface OSPF priority first and the router ID as the tiebreaker.
| Decision step | Criterion | Result |
|---|---|---|
| 1 | Eligibility | An interface with priority 0 cannot become DR or BDR. |
| 2 | Interface OSPF priority | The highest eligible priority is preferred. |
| 3 | Router ID | If priorities tie, the highest router ID wins. |
| 4 | Role assignment | The best candidate becomes DR and the next best candidate becomes BDR. |
The default OSPF interface priority is 1. Priority is configured per interface, not globally for every interface participating in OSPF.
An interface priority of 0 makes that router ineligible for both roles. The router can still participate in OSPF as a DROTHER and can still form full adjacencies with the DR and BDR.
The router ID is a 32-bit OSPF identifier. It may look like an IPv4 address, but it is an OSPF identifier rather than necessarily the address used to send packets. When priority values are equal, the highest router ID wins the DR election and the next-highest eligible router ID becomes BDR.
Example: Default Election
Assume four routers share one OSPF area 0 Ethernet LAN. All interfaces have the default priority of 1, and their router IDs are:
Router A: 100.0.0.0
Router B: 90.0.0.0
Router C: 70.0.0.0
Router D: 60.0.0.0Because all priorities tie, router ID decides the result:
- Router A, with router ID 100.0.0.0, becomes DR.
- Router B, with router ID 90.0.0.0, becomes BDR.
- Routers C and D become DROTHER routers.
Non-Preemptive Election Behavior
OSPF DR and BDR elections are non-preemptive. After roles are established, adding a router with a higher priority does not normally replace the current DR. Changing a DROTHER's priority also does not immediately make it DR.
A new election can occur after events such as:
- DR failure and dead-timer expiration.
- BDR promotion after DR failure.
- An interface reset.
- An OSPF process reset.
- Clearing OSPF neighbors.
Forcing a new election disrupts adjacencies and can temporarily affect routing convergence, so it should be planned and approved in a production network.
Multicast Addresses on Broadcast Networks
| Address | Name | Typical sender | Typical receivers | Purpose |
|---|---|---|---|---|
| 224.0.0.5 | AllSPFRouters | DR or another OSPF router | All OSPF routers on the segment | Distribute OSPF packets to every OSPF router |
| 224.0.0.6 | AllDRouters | DROTHER router | DR and BDR | Send link-state updates toward the designated routers |
On a broadcast segment, a DROTHER that has new link-state information sends the LSU toward 224.0.0.6, the AllDRouters address. The DR then floods the information to 224.0.0.5, the AllSPFRouters address.
This behavior prevents every router from independently flooding every other router. The exact packet behavior differs on NBMA networks because NBMA does not provide native broadcast or multicast delivery.
What Happens When the DR Fails?
- Routers stop receiving valid Hello packets from the DR.
- The dead interval expires.
- OSPF declares the DR unavailable.
- The BDR, which has been fully synchronized, takes over as DR.
- OSPF elects a new BDR from the remaining eligible routers.
The BDR is ready for this transition because it maintains full adjacencies and synchronized LSAs during normal operation. It does not normally originate the segment's network LSA while the DR remains active; that responsibility begins when it becomes DR.
Verifying DR and BDR Roles
Using show ip ospf neighbor
show ip ospf neighborUse this command to inspect neighbor router IDs, priority, state, dead timer, source address, and local interface. The State column identifies the neighbor's role:
- FULL/DR: the neighbor is the DR and the adjacency is full.
- FULL/BDR: the neighbor is the BDR and the adjacency is full.
- FULL/DROTHER: the neighbor is a DROTHER with which a full adjacency exists, commonly from the perspective of a DR or BDR.
- 2WAY/DROTHER: the neighbor is another non-designated router; this is normally expected on a broadcast or NBMA segment.
Using show ip ospf interface
show ip ospf interface FastEthernet0/0This command shows the local interface's OSPF network type, configured priority, local state, DR address, and BDR address. It helps distinguish an election problem from a network-type issue.
Using show ip ospf interface brief
show ip ospf interface briefThis is a concise way to review OSPF-enabled interfaces and their local roles. For detailed timers, priorities, and designated-router information, use the interface-specific command.
| Output field | Meaning | Example values | What to check |
|---|---|---|---|
| Neighbor ID | Neighbor's OSPF router ID | 100.0.0.0 | Remember that this is not necessarily the neighbor's interface IP address. |
| Priority | Neighbor interface's election priority | 0, 1, 200 | Priority 0 means the neighbor is ineligible. |
| State | Adjacency state and neighbor role | FULL/DR, FULL/BDR, 2WAY/DROTHER | Confirm full adjacency to the DR and BDR. |
| Dead Time | Time remaining before the neighbor is declared down without a Hello | 00:00:36 | Check for timer mismatches or unstable connectivity. |
| Address | Neighbor source address on the segment | 192.0.2.2 | Confirm the expected subnet and interface. |
| Interface | Local interface used for the neighbor relationship | FastEthernet0/0 | Confirm the correct OSPF interface and network type. |
Influencing the Election
Set OSPF priority on the shared-segment interface. A higher value is preferred over a lower value.
interface FastEthernet0/0
ip ospf priority 200In this example, the router is a strong DR or BDR candidate on FastEthernet0/0. The setting applies only to that interface.
To prevent a low-capacity router from becoming DR or BDR while allowing it to participate as a DROTHER, configure priority 0:
interface FastEthernet0/0
ip ospf priority 0After changing priority, existing roles usually remain because the election is non-preemptive. To intentionally force neighbor re-formation during a maintenance window, Cisco IOS commands may include:
clear ip ospf neighbor
clear ip ospf processPractical Election Examples
Priority Overrides Router ID
Suppose a router has a lower router ID than the other candidates but should be preferred as DR. Configure its shared-LAN interface with priority 200 and leave the other candidates at priority 1:
interface FastEthernet0/0
ip ospf priority 200The priority of 200 wins before router ID is considered. Router ID matters only when eligible candidates have equal priorities.
Topology Change Propagation
If a DROTHER loses a directly connected subnet, it creates updated link-state information and sends the LSU toward 224.0.0.6. The DR receives it and floods the information to 224.0.0.5. The BDR also receives the update and remains synchronized, ready to become DR if necessary.
Non-Preemption After a Change
If a current DROTHER is configured with a higher priority after the DR and BDR already exist, the current DR does not immediately give up its role. Verify the current roles with show ip ospf neighbor and the configured values with show ip ospf interface. If a role change is required, schedule a controlled election reset.
Troubleshooting DR and BDR Problems
2WAY/DROTHER Is Seen Instead of FULL
This is normally correct. Two DROTHER routers on a broadcast or NBMA segment do not usually form full adjacencies with each other. Confirm that each one has FULL adjacency with the DR and BDR.
show ip ospf neighborThe Highest-Priority Router Did Not Become DR
The likely cause is non-preemption: the election completed before the priority was changed. Check current roles and priorities, then plan a controlled reset if a new election is required.
show ip ospf neighbor
show ip ospf interface FastEthernet0/0An Expected Candidate Is Always DROTHER
Check whether the interface priority is 0. Also check the network type. Point-to-point and point-to-multipoint interfaces do not elect DR or BDR roles.
show ip ospf interface FastEthernet0/0No DR or BDR Appears
This may be expected on a point-to-point or point-to-multipoint link. Verify the network type before treating the absence of a DR or BDR as a fault.
A Neighbor Never Reaches FULL
Check for an area mismatch, authentication mismatch, subnet mismatch, timer mismatch, passive-interface configuration, OSPF network-type mismatch, or failed bidirectional Hello communication.
show ip ospf neighbor
show ip ospf interface FastEthernet0/0
show running-config interface FastEthernet0/0
show ip protocolsThe BDR Does Not Appear to Flood Updates Like the DR
This is normal while the DR is operating. The BDR is a synchronized standby. Confirm that it has a full adjacency and is listed as BDR; it becomes the active DR only after the current DR fails.
Exam-Relevant Summary
- Broadcast and NBMA multiaccess networks elect a DR and BDR.
- Point-to-point and point-to-multipoint networks do not elect them.
- DROTHER routers form full adjacencies with the DR and BDR.
- Two DROTHER routers normally remain 2-Way with each other.
- The default interface priority is 1; priority 0 prevents DR/BDR eligibility.
- Higher priority wins; the highest router ID breaks equal-priority ties.
- DR/BDR elections are non-preemptive.
- The DR originates the Type 2 Network LSA for the shared segment.
- The BDR is fully synchronized and promotes when the DR fails.
- 224.0.0.6 reaches the DR and BDR; 224.0.0.5 reaches all OSPF routers.
- Router ID, not necessarily an interface IP address, is the election tiebreaker.
For related study, see OSPF Designated Router and Backup Designated Router.