OSPF : Why are DR and BDR needed?

 In Open Shortest Path First (OSPF), DR (Designated Router) and BDR (Backup Designated Router) are elected to reduce the amount of OSPF traffic on multi-access networks like Ethernet.

Why are DR and BDR needed?

Imagine 5 routers connected to the same Ethernet switch.

Example: Ethernet LAN (Multi-access)

          Switch
      +-----+-----+
      |     |     |
     R1    R2    R3
      |     |     |
     R4    R5

All five routers are connected to the same Ethernet switch and are on the same subnet

Without a DR:

• Every router forms a neighbor relationship with every other router.
• Number of adjacencies = n(n−1)/2

For 5 routers:

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

That's 10 adjacencies.

With a DR:

        R2
         |
R3 ---- DR ---- R4
         |
        R5
         |
        R1

Each router forms a full adjacency only with the DR (and BDR), greatly reducing overhead.

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DR and BDR election

The election is based on:

1. Highest OSPF interface priority
2. If priorities are equal, highest Router ID

Example:

Router	Priority	Router ID
R1	1	1.1.1.1
R2	100	2.2.2.2
R3	1	3.3.3.3

Result:

• DR: R2 (highest priority)
• BDR: R3 (highest Router ID among the remaining routers)

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Interface priority

Check or configure it:

interface GigabitEthernet0/0
 ip ospf priority 50

Default priority:

1

To prevent a router from becoming DR or BDR:

interface GigabitEthernet0/0
 ip ospf priority 0

A priority of 0 means the router is not eligible for election.

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Router ID tie-breaker

If all routers have the default priority of 1:

Router	Router ID
R1	1.1.1.1
R2	2.2.2.2
R3	3.3.3.3

Result:

• DR: R3
• BDR: R2

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Is the election preemptive?

No.

Suppose:

1. R1 becomes DR.
2. R2 becomes BDR.
3. Later, R3 (with a higher priority) joins the network.

R3 does not replace the current DR. It remains a DROther router until another election occurs (for example, if the DR fails or OSPF is reset on the segment).

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If the DR fails

Suppose:

• DR = R1
• BDR = R2
• DROther = R3

If R1 goes down:

• R2 is immediately promoted to DR.
• A new BDR is elected from the remaining eligible routers (R3, if eligible).

This minimizes disruption because the BDR is already synchronized with the network.

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OSPF neighbor states during election

Routers typically progress through these states:

Down
 ↓
Init
 ↓
2-Way
 ↓
ExStart
 ↓
Exchange
 ↓
Loading
 ↓
Full

On a broadcast network:

• Routers usually reach the Full state only with the DR and BDR.
• Between two DROther routers, the neighbor state typically remains 2-Way because they do not form full adjacencies with each other.

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On which network types is DR/BDR used?

Network Type	DR/BDR Election
Broadcast (Ethernet)	✅ Yes
Non-Broadcast Multi-Access (NBMA)	✅ Yes
Point-to-Point	❌ No
Point-to-Multipoint	❌ No