OSPF DR and BDR

OSPF needs to elect a DR (Designated Router) and a BDR (Backup Designated Router) to function efficiently on multi-access networks.

Why are OSPF DR and BDR necessary?

On multi-access segments (such as Ethernet), many routers can be connected to the same network, as shown below. When each router tries to form OSPF neighbor adjacencies with every other router, the number of relationships increases rapidly.

Figure 1 – Full Mesh OSPF Adjacency Without DR/BDR on a Multi-Access Network

The number of adjacencies grows according to the formula:
n(n – 1) / 2, where n is the number of routers.

In the example above, we have 4 routers:
4(4 – 1) / 2 = 6 adjacencies

Now imagine a larger segment with 10 routers:
10(10 – 1) / 2 = 45 adjacencies

This quickly becomes inefficient. More routers means:

• More OSPF neighbors to maintain

• More LSAs exchanged between devices

• More CPU and memory consumed to keep OSPF running

That’s exactly why OSPF DR and BDR were introduced to reduce adjacency overload and improve scalability on multi-access networks.

Imagine a network without a DR or BDR.
When a router like R1 joins the segment, it sends LSAs to 224.0.0.5 (All OSPF Routers).

Figure 2 - R1 sends LSA to all Router OSPF (224.0.0.5)

All other routers on the segment (R2, R3, R4) receive this LSA, acknowledge it, and then re-flood it to the others again to 224.0.0.5.

Figure 3 - Routers re-flood the LSA to all Router OSPF (224.0.0.5)

This creates a storm of redundant LSAs, especially when multiple routers advertise new information. It wastes bandwidth and CPU.

To solve this issue, OSPF introduces the concept of a DR (Designated Router) and BDR (Backup Designated Router).

The concept is very simple, the DR collects all the LSA and sends the LSA one time to all other routers. The BDR act as a Backup when the DR fails and takes its place.

With a DR and BDR elected:

• R1 forms adjacencies with the DR and BDR

• When R1 generates an LSA, it sends it only to 224.0.0.6 (All Designated Router).

Figure 4 - R1 sends LSA only to DR and BDR using 224.0.0.6

Then:

• The DR forwards the LSA to all other routers using 224.0.0.5

• The BDR is ready to take over if the DR fails

Figure 5 - DR sends LSA to all other routers

The LSA is flooded only one time and every router learns about it without unnecessary duplication.
To achieve this behavior, OSPF performs a DR/BDR election.

In OSPF, the DR/BDR election takes place on multi-access networks (like Ethernet), not on point-to-point links.

Where Does the DR/BDR Election Happen?

This election happens after the 2-Way state during OSPF neighbor formation.

Figure 6 - How OSPF Reaches the DR/BDR Election Step

The Election only occurs on multi-access networks. If the interface is configured as a point-to-point link, there’s no DR and BDR election.

Each OSPF router checks the network type on its interface and determines if it needs to participate in the DR/BDR election.

DR and BDR Election Rules

The DR and BDR routers are selected based on OSPF priority and Router ID:

Election rules

1. The router with the highest priority becomes DR

2. The second-highest becomes BDR

3. If equal, the highest Router ID wins

4. A router with priority 0 is not eligible

Figure 7 - OSPF Election Roles (DR, BDR and DROTHER)