ExStart is the OSPF neighbor state where two routers choose which one leads the database description (DBD) exchange. The router with the higher router ID becomes Leader; the lower becomes Follower. This exchange moves the neighbors toward a full adjacency. section 18.3.1
Remember the sequence and terminology:
- Routers first reach 2-Way, where they acknowledge each other but have not exchanged routing information. section 18.3.1
- In ExStart, they select the DBD exchange leader using router ID. section 18.3.1
- In Exchange, they send DBDs containing summaries of their link-state databases; Loading then requests specific LSAs. section 18.3.1
- Cisco IOS may still use the older Master/Slave terminology for Leader/Follower. section 18.3.1
R1 and R2
After R1 and R2 acknowledge each other in 2-Way, they enter ExStart. Comparing router IDs determines the leader; the actual DBD exchange follows in Exchange. section 18.3.1
