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| 6.2 |  | OSPF Operation |
| 6.2.4 |  | Step 3: Discover routes |

On a multiaccess network, the exchange of routing information occurs between the DR or BDR and every other router on the network. As the DR and BDR on the 10.5.0.0 /16 network, RTA and RTB will exchange link-state information.

Link partners on a point-to-point or point-to-multipoint network also engage in the exchange process. That means that RTB and RTC will share link-state data.

However, who goes first? This question is answered in the first stage of the Exchange process, the ExStart state. The purpose of ExStart is to establish a master/slave relationship between the two routers. The router that announces the highest router ID in the Hello packet acts as master. The master router orchestrates the exchange of link-state information, while the slave router responds to prompts from the master. RTB engages in this process with both RTA and RTC.

After the routers define their roles as master and slave, they enter the Exchange state. The master leads the slave through a swap of DBDs that describe the link-state database in limited detail for each router. These descriptions include the link-state type, the address of the advertising router, the cost of the link, and a sequence number.

The routers acknowledge the receipt of a DBD by sending an LSAck (Type 5) packet, which echoes back the sequence number of the DBD. Each router compares the information that it receives in the DBD with the information that it already has. If the DBD advertises a new or more up-to-date link state, the router will enter the Loading state by sending an LSR (Type 3) packet about that entry. In response to the LSR, a router sends the complete link-state information, using an LSU (Type 4) packet. LSUs carry LSAs.

With the Loading state complete, the routers have achieved full adjacency and entered into the Full state. Figure shows that RTB is now adjacent to RTA and to RTC. Adjacent routers must be in the Full state before they can create their routing tables and route traffic. At this point, the neighbor routers should all have identical link-state databases.