Building the Routing Table on a New OSPF Router

Five packets are used to build the routing table for the first time:

• Hello protocol—Is used to find neighbors and to determine the designated and backup designated router. The continued propagation of the Hello protocol maintains the transmitting router in the topology database of those that hear the message.

• Database descriptor—Is used to send summary information to neighbors to synchronize topology databases.

• Link-state request—Works as a request for more detailed information, which is sent when the router receives a database descriptor that contains new information.

• Link-state update—Works as the link-state advertisement (LSA) packet issued in response to the request for database information in the link-state request packet. The different types of LSA are described in Chapter 6, in the section "The Link-State Advertisements."

• Link-state acknowledgement—Acknowledges the link-state update.

Consider the case of when a router joins the OSPF network for the first time. In Figure 5-7, the 2500 router in Building A at the San Francisco campus has just been connected.

The next sections detail what happens when a router joins a network.

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