OSPF DR and BDR Explained: Designated Router Election Process

The Solution: DR and BDR

OSPF solves this with two special routers:

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:

Router	Priority	Router ID	Result
R1	100	1.1.1.1	DR (highest priority)
R2	50	2.2.2.2	BDR (second-highest priority)
R3	1	3.3.3.3	DROther
R4	1	4.4.4.4	DROther

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:

R2 and R3 come online. R3 becomes DR (higher Router ID).
R1 (priority 100) boots up and joins.
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 (Article 17)

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

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 next.

Screenshot Suggestions:

Topology diagram showing DR, BDR, and DROthers with adjacency lines
show ip ospf neighbor output with annotations for FULL/DR, FULL/BDR, 2WAY/DROTHER
show ip ospf interface output highlighting State and Priority
Election flowchart: Priority comparison → Router ID tiebreaker

Internal Links:

← OSPF Neighbor States Explained (Article 3)
→ How OSPF Calculates Metric and Cost (Article 6)
→ OSPF Network Types Explained (Article 17)
→ OSPF Passive Interfaces (Article 9)

OSPF DR and BDR: What They Are and Why They Matter