In this lesson, I have to explain to you about the DR (Designated Router) and BDR (Backup Designated Router) election process. Let’s say that we have this scenario: we have one switch, and inside this switch, we have 8 routers. These routers are all on OSPF Area 0, which is the backbone area for OSPF.

Once we enable OSPF on all these routers, they will form neighborships and start sending LSAs (Link-State Advertisements) to each other. In a broadcast network, every router sends LSAs to every other router.

This means that if there are n routers, the formula for calculating the total number of LSAs exchanged is:

LSAs=n×(n−1)2

For example, if we have 8 routers:

LSAs=8×72=28\

LSAs=28×7​=28

So, 28 LSAs are exchanged. Now imagine if we had 40 routers:

LSAs=40×392=780

LSAs=240×39​=780

This is a huge number, and since LSAs are multicast, it puts a significant burden on the network.

The Solution: DR and BDR

To solve this problem, OSPF uses a mechanism to elect a Designated Router (DR) and a Backup Designated Router (BDR).

Without DR and BDR

In a scenario without DR and BDR, all routers form neighborships with every other router, resulting in a full mesh topology. This is inefficient and leads to excessive LSA flooding.