Hi,

An example would be overly complicated (OSPF is not the easiest protocol to go through) but think of this:

When you want to synchronize your database to your neighbor's database, you do not need to transfer those items you both have - while not harmful, it would be a waste of effort and bandwidth. If you want to know everything you neighbor knows, you only want to download those items that are present in the neighbor's database and missing from your database, and the items that are more recent in the neighbor's database than in your database.

To make the list of items that are missing from your database or are outdated, you only need to receive an inventory of the neighbor's database - not its complete contents, just the list of items including their identifiers and sequence numbers to know which are missing and which are more recent. This is the purpose of the Exchange stage - to exchange the inventory information about the database contents between both routers.

As you process this inventory, you might indeed find out that some of the items claimed by your neighbor are missing in your database, or that the neighbor has a more recent item. You want to download these, and only these, items from the neighbor to make the synchronization as efficient as possible. This is the purpose of the Loading stage - you explicitly ask for those items you find missing or outdated in your database, and you expect your neighbor to send them to you. After receiving all requested items, you move from the Loading to the Full stage.

These processes occur on both synchronizing routers in parallel. After both routers have exchanged their database inventories during the Exchange stage, and transmitted the missing or more recent entries during the Loading stage, they each move their neighbor from the Loading to the Full state, knowing that now, both routers have the same information.

Best regards,
Peter