This is a text description of Figure 2-10, "Removing Obsolete Nodes". It illustrates the removal of obsolete nodes from a topology, as explained in text before and after the illustration. In this case, only one node (N2) is removed, because the other nodes do not satisfy all criteria for obsolete nodes.
In the top part of the illustration (Before Removing Obsolete Nodes), edge E1 is between nodes N1 and N2, and edge E2 is between nodes N2 and N3. In the bottom part of the illustration (After Removing Obsolete Nodes), edge E1 is between nodes N1 and N3, and edge E2 does not exist because it has been removed.