A change in the network topology also necessitates information to be propagated through the network. For example, the rest of the network notices that a new router has joined their ranks.
A particular router that notices a change floods the area with the update so that all routers can alter their routing tables to reflect the most current and accurate information.
Different techniques are used for these different routing table requirements. Essentially, the difference between the two techniques is simple:
• If a new router connects to a network, it will find a neighbor using the Hello protocol and will exchange routing information.
• If a change occurs in an existing network, the router that sees the change will flood the area with the new routing information.
NOTE Both of these events must occur as stated because, although the new router must learn the network topology, its addition is a change to the rest of the network.
These two requirements for updating the routing table use different technologies and OSPF protocols. These technologies and protocols are often confused, so it is worth a moment to distinguish them. Understanding the distinction makes the OSPF operation much clearer.
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