4.2 Steps to OSPF Operation
4.2.4 Step 3 : Discovering routes
After the DR and BDR have been elected, the routers are considered to be in the Exstart state, as shown in the Figure . They are ready to discover the link-state information about the internetwork and create their link-state databases.
Lab Activity
  In this lab, you will learn how to observe the DR and BDR selection process.

The process used to discover the network routes is called the exchange process, and is performed to advance the routers to a Full state of communication. After adjacent routers are in a Full state, they do not modify the exchange protocol unless the Full state changes. The exchange process operates as follows:

  1. In the Exstart state,  the DR and BDR establish adjacencies with each router in the network. During this process, a master-slave relationship is created between each router and its DR/BDR. The router that has the higher router ID acts as the master. 
    Note that link-state information is exchanged and synchronized only between the DR/BDR and the routers to which they have established adjacencies. Having the DR represent the network in this capacity reduces the amount of routing update traffic.
  2. The master and slave routers exchange one or more database description packets (DDPs or DBDs). This is referred to as the Exchange state.
    A DDP includes an LSA (link-state advertisement) that describes the router's link-state database. Each database description LSA includes a variety of information such as link-state type, the address of the advertising router, the cost of the link, and the sequence number. The sequence number is a router's way of determining the order in which the link-state information was received. The initial sequence number used by the adjacent routers to start keeping track of the LSA sequence is defined by the master.
  3. When the slave router receives the DDP, it does the following:
  • Acknowledges the receipt of the DDP by echoing the link-state entry sequence numbers in a link-state acknowledgment (LSAck) packet, as shown in the figure..
  • Compares the information it received with the information it has. Remember, the initial entries put into the link-state database are from the adjacencies database. If the DDP has a more up-to-date link-state entry, then the slave router sends a link-state request (LSR) to the master router.
  • The master router responds with the complete information about the requested entry in a link-state update (LSU) packet. Again, the slave router sends an LSAck when the LSU is received. The process of sending LSRs is referred to as the Loading state.
  1. All routers add the new link-state entries into their link-state database.
  2. After all LSRs have been updated for a given router, adjacent routers are considered synchronized in a Full state. The routers must be in the Full state before they can route traffic. At this point, the routers should all have identical link-state databases.
Lab Activity
  In this lab, you will learn how to force the DR and BDR election process.