In the Cisco Press book "Authorized Self-Study Guide BSCI, 3rd Ed.", on page 78, we can see the figure 3-4 showing the initial neighbor discovery process. This process is described step by step on page 79 and it seems the text description does not match the figure.
On the figure, we can see that Router A sends a Hello message to Router B and then Router B sends a Hello message to Router A back. Router B immediately sends the Update messages with its routes.
In the text, point 2 is very unclear to me. It is mentionned that
"Router B receiving the hello packet on one interface reply with update packets that contain all the routes it has in its routing table, except those learned through that interface (because of the split horizon rule). Router B sends an update packet to Router A, but a neighbor relationship is not established until Router B sends a hello packet to Router A. The update packet contains information about the routes that Router B is aware of, including the metric that the neighbor is advertising for each destination."
All this point is very very confusing to me. How does exactly Router B behaves when he receives a Hello packet from a router he has not formed an adjacency with ? Does it first send a Hello packet back (like in the figure, this would be a reasonable answer for me) or does it send an Update packet back without worrying about sending back an Hello ? In that case, why does it stupidly send an Update while it has not fully established the neighborship ?
Thank you very much for your answers.