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# OSPF DR and BDR: What They Are and Why They Matter
- URL: https://www.pinglabz.com/ospf-dr-bdr-designated-router/
- Published: 2026-03-24T03:16:24.000Z
- Updated: 2026-08-02T05:13:19.000Z
- Description: On multi-access OSPF networks, the DR and BDR centralize LSA exchange so routers do not flood 45 sessions across 10 neighbors. A practical guide to what the DR and BDR do and why OSPF design depends on them.
- Author: Jaime
- Tags: OSPF, #Import 2026-08-01 19:54

## The Solution: DR and BDR

OSPF solves this with two special routers:

If you want the bigger picture first, the [OSPF complete guide](https://www.pinglabz.com/ospf/) covers the architecture this article plugs into.

### Designated Router (DR)

**What it is:**  
The central point for LSA exchange on a multi-access network.

**What it does:**

- Forms full adjacencies with all other routers
- Generates the **Type 2 Network LSA** for the segment
- Receives LSA updates from all routers and redistributes them

**Multicast address:** 224.0.0.6 (AllDRouters)

### Backup Designated Router (BDR)

**What it is:**  
The backup to the DR.

**What it does:**

- Forms full adjacencies with all other routers (just like the DR)
- Listens to all LSA traffic
- Takes over if the DR fails (no election needed - it's already synchronized)

**Why it exists:**  
Provides **instant failover** without needing to re-elect and re-synchronize.

### DROther

**What it is:**  
All routers that are *not* the DR or BDR.

**What they do:**

- Form full adjacencies with the DR and BDR
- Form **2-Way adjacencies** with each other (no LSA exchange)
- Send LSA updates to the DR (224.0.0.6)

## How DR/BDR Reduces Overhead

### Without DR/BDR (5 routers):

- Full adjacencies: **10**
- LSA flooding: Every router floods to every other router

### With DR/BDR (5 routers):

- Full adjacencies: **8** (DR ↔ all 5, BDR ↔ all 5, minus duplicates)
- LSA flooding: DROthers send to DR, DR redistributes
- Result: **20% fewer adjacencies**, far less flooding

## DR/BDR Election Process

### When Election Happens

- When the first two routers come online on a segment
- When the DR fails and there's no BDR
- **NOT when a router with higher priority joins** (election is non-preemptive)

### Election Rules

**Step 1: Check OSPF Priority**  
The router with the **highest OSPF priority** becomes the DR.  
The router with the **second-highest priority** becomes the BDR.

**Default priority:** 1  
**Range:** 0-255  
**Priority 0:** Router will never become DR or BDR (use this for weak routers)

**Step 2: Tiebreaker (Router ID)**  
If priorities are equal, the router with the **highest Router ID** wins.

### Example Election

**Scenario:**  
4 routers on the same Ethernet segment:

100

RouterR1

Router ID1.1.1.1

Result**DR** (highest priority)

50

RouterR2

Router ID2.2.2.2

Result

**BDR** (second-highest priority)

1

RouterR3

Router ID3.3.3.3

ResultDROther

1

RouterR4

Router ID4.4.4.4

ResultDROther

Even though R4 has the highest Router ID (4.4.4.4), R1 becomes DR because of **priority**.

### Election is Non-Preemptive

**Key point:**  
Once a DR is elected, it stays the DR until it goes down - **even if a router with higher priority joins later**.

**Example:**

1. R2 and R3 come online. R3 becomes DR (higher Router ID).
2. R1 (priority 100) boots up and joins.
3. **R3 stays DR.** R1 does not take over.

**To force a new election:**  
You must clear the OSPF process on the DR:

```
Router# clear ip ospf process

```

⚠️ **Warning:** This disrupts OSPF on that router.

## Controlling the DR/BDR Election

### Set OSPF Priority

**Default:** 1  
**To make a router the DR:** Set priority higher than other routers  
**To prevent a router from becoming DR/BDR:** Set priority to 0

**Configuration:**

```
Router(config)# interface gi0/0
Router(config-if)# ip ospf priority 100

```

**Verification:**

```
Router# show ip ospf interface gi0/0
GigabitEthernet0/0 is up, line protocol is up
  Internet Address 192.168.1.1/24, Area 0
  Process ID 1, Router ID 10.0.0.1, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DR, Priority 100
  Designated Router (ID) 10.0.0.1, Interface address 192.168.1.1
  Backup Designated router (ID) 10.0.0.2, Interface address 192.168.1.2
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

```

**Key fields:**

- **State:** DR (this router is the DR)
- **Priority:** 100
- **Designated Router:** Shows the DR's Router ID and IP
- **Backup Designated Router:** Shows the BDR's Router ID and IP

### Example: Ensuring Core Router is DR

**Scenario:**  
You have a core router (R1) and three access-layer routers (R2, R3, R4) on the same VLAN.

**Goal:**  
Make R1 the DR so it handles LSA distribution (it has more CPU/bandwidth).

**Configuration (R1):**

```
Router(config)# interface vlan 10
Router(config-if)# ip ospf priority 200

```

**Configuration (R2, R3, R4):**

```
Router(config)# interface vlan 10
Router(config-if)# ip ospf priority 50

```

**Result:**  
R1 becomes DR (priority 200).  
The router with priority 50 and highest Router ID becomes BDR.

### Example: Preventing a Router from Becoming DR

**Scenario:**  
R5 is a low-end router. You don't want it handling DR responsibilities.

**Configuration (R5):**

```
Router(config)# interface gi0/0
Router(config-if)# ip ospf priority 0

```

**Result:**  
R5 will **never** become DR or BDR on this segment.

## DR/BDR on Different Network Types

### Broadcast (Ethernet, default)

- **DR/BDR:** Yes
- **Full adjacencies:** DR ↔ all, BDR ↔ all
- **DROther adjacencies:** 2-Way with each other

### Point-to-Point

- **DR/BDR:** No (not needed - only 2 routers)
- **Full adjacencies:** Both routers always Full

### NBMA (Frame Relay, ATM)

- **DR/BDR:** Yes
- **Complication:** NBMA doesn't support multicast, so you need manual neighbor statements

### Point-to-Multipoint

- **DR/BDR:** No
- **Full adjacencies:** All routers form full adjacencies

**Learn more:** [OSPF Network Types Explained](https://www.pinglabz.com/ospf-network-types-explained/)

## Verifying DR/BDR Status

### Check Interface State

```
Router# show ip ospf interface gi0/0
GigabitEthernet0/0 is up, line protocol is up
  Process ID 1, Router ID 10.0.0.1, Network Type BROADCAST
  Transmit Delay is 1 sec, State DR, Priority 100
  Designated Router (ID) 10.0.0.1, Interface address 192.168.1.1
  Backup Designated router (ID) 10.0.0.2, Interface address 192.168.1.2

```

**Key fields:**

- **State:** DR / BDR / DROTHER
- **Priority:** Current OSPF priority
- **Designated Router:** Who the DR is
- **Backup Designated Router:** Who the BDR is

### Check Neighbors and Their Roles

```
Router# show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
10.0.0.2        100   FULL/BDR        00:00:35    192.168.1.2     Gi0/0
10.0.0.3        1     FULL/DROTHER    00:00:31    192.168.1.3     Gi0/0
10.0.0.4        1     2WAY/DROTHER    00:00:38    192.168.1.4     Gi0/0

```

**What you see:**

- **10.0.0.2:** BDR, Full adjacency
- **10.0.0.3:** DROther, Full adjacency (this router is the DR)
- **10.0.0.4:** DROther, 2-Way adjacency (normal - DROthers don't form Full with each other)

**Interpretation:**  
The router running this command is the **DR** because:

- It has FULL adjacencies with all routers (including DROthers)
- Other DROthers are at 2-Way with each other

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## Common DR/BDR Issues

### Problem 1: Wrong Router is DR

**Symptom:**  
A low-power access switch became DR instead of the core router.

**Cause:**

- The access switch booted first and won the election
- Non-preemptive election means it stayed DR

**Solution 1 (Temporary):**  
Clear OSPF process on the current DR:

```
Router# clear ip ospf process

```

A new election happens. The router with highest priority wins.

**Solution 2 (Permanent):**  
Set OSPF priorities correctly *before* deployment:

```
! Core router (should be DR)
Router(config-if)# ip ospf priority 200

! Access switches (should be DROther)
Router(config-if)# ip ospf priority 10

```

### Problem 2: No BDR Elected

**Symptom:**  
Only one router on the segment, so there's a DR but no BDR.

**Cause:**  
BDR election requires at least 2 routers.

**Impact:**  
If the DR fails, a new election must happen (slower failover).

**Solution:**  
Ensure at least 2 routers are on multi-access segments in production.

### Problem 3: DR/BDR Flapping

**Symptom:**  
DR keeps changing.

**Cause:**

- Current DR keeps rebooting
- OSPF priority misconfigured (multiple routers fighting for DR)
- Network instability

**Solution:**

- Check DR router health (CPU, memory, crashes)
- Verify OSPF priority consistency
- Check for duplicate Router IDs

## DR/BDR Best Practices

### 1\. Set Priorities Explicitly in Production

Don't rely on default priority (1) and Router ID tiebreakers. Explicitly configure:

- **Core/distribution routers:** Priority 100-200
- **Access routers:** Priority 10-50
- **Weak devices:** Priority 0

### 2\. Make the Fastest Router the DR

The DR handles LSA redistribution. Make it the router with:

- Most CPU
- Most bandwidth
- Best reliability

### 3\. Use Priority 0 on User-Facing Routers

If a router only needs OSPF for reachability (not routing), set priority 0:

```
Router(config-if)# ip ospf priority 0

```

### 4\. Document Your DR/BDR Design

In the network documentation, note:

- Which routers should be DR/BDR
- OSPF priorities per segment

### 5\. Avoid Changing Priorities in Production

Changing OSPF priority won't trigger an immediate re-election. You'd need to clear the OSPF process, which disrupts routing.

**Best practice:**  
Set priorities correctly during initial deployment.

## DR/BDR in Multi-Area Environments

**Key point:**  
DR/BDR election is **per-segment**, not per-area.

**Example:**  
A router can be:

- DR on VLAN 10 (Area 0)
- DROther on VLAN 20 (Area 10)

Each segment elects its own DR/BDR independently.

## Summary: DR/BDR Checklist

Now you know:

✅ **Why DR/BDR exist** \- Reduce adjacencies and LSA flooding on multi-access networks  
✅ **How election works** \- Highest priority, then highest Router ID  
✅ **Election is non-preemptive** \- DR stays DR until it fails  
✅ **How to control election** \- Set OSPF priority per interface  
✅ **When DR/BDR are used** \- Broadcast and NBMA networks only  
✅ **Common issues** \- Wrong router elected, DR flapping

**Next Step:**  
DR/BDR is one piece of OSPF efficiency. The other is **OSPF cost** \- the metric that determines the best path. Read [**How OSPF Calculates Metric and Cost**](https://www.pinglabz.com/ospf-metric-and-cost-calculation/) next.

**Internal Links:**

- ← [OSPF Neighbor States Explained](https://www.pinglabz.com/ospf-neighbor-states-explained/)
- → [How OSPF Calculates Metric and Cost](https://www.pinglabz.com/ospf-metric-and-cost-calculation/)
- → [OSPF Network Types Explained](https://www.pinglabz.com/ospf-network-types-explained/)
- → [OSPF Passive Interfaces](https://www.pinglabz.com/ospf-passive-interface/)