OSPF DR/BDR Election explained

Hello Sims

When a new OSPF neighbor relationship is created, the first thing that happens is an exchange of information to determine who is the master and who is the slave. Each OSFP neighbor relationship between two OSPF routers has a master and a slave. The purpose of this determination is to decide who speaks first.

The master/slave negotiation in OSPF happens during the exstart state : both neighbors will claim to be the master by sending an empty database description packet with the Master Slave bit (MS-bit) set to one indicating that both routers believe they are the master. The neighbor with the lower Router ID will become the slave and will reply with a DD packet in which the MS-bit is zero and the DD sequence number is set to the master’s sequence number.

From there on in, the master will always speak first in subsequent communications.

I hope this has been helpful!

Laz