In the OSPF 'Ex Start' state, the master begins the Database Description (DBD) packets exchange. Following this, through the 'Loading' state, Link State Requests (LSR) and Link State Updates (LSU) are exchanged to get more information on the routes in the DBD. The DBD contains abbreviated list of the sending routers Link State Database (LSDB) and is used by the receiving router to check against its own LSDB. Receiving routers can request more information about any entry in the DBD by sending a LSR, for example for new entries or entries with a higher sequence number. The LSU's are used to reply to LSR's. Link State Ack's are used to confirm receipt of LSUs.

• Source: From my own CCNA study, thanks for the revision practice! (Maybe try learningnetwork.cisco.com for more formal explanations.)

In the OSPF 'Ex Start' state, the master begins the Database Description (DBD) packets exchange. Following this, through the 'Loading' state, Link State Requests (LSR) and Link State Updates (LSU) are exchanged to get more information on the routes in the DBD. The DBD contains abbreviated list of the sending routers Link State Database (LSDB) and is used by the receiving router to check against its own LSDB. Receiving routers can request more information about any entry in the DBD by sending a LSR, for example for new entries or entries with a higher sequence number. The LSU's are used to reply to LSR's. Link State Ack's are used to confirm receipt of LSUs.

• Source: From my own CCNA study, thanks for the revision practice! (Maybe try Cisco Learning Network for more formal explanations.)