Router One assumes that Router Three would learn about Network A directly from Router Two.
Poison reverse is another way of avoiding routing loops.
Let's say the routers in Figure 4a have poison reverse enabled. When Router One learns about Network A from Router Two, it advertises Network A as unreachable through its link to Routers Two and Three. Router Three, if it shows any path to Network A through Router One, removes that path because of the unreachable advertisement. Let's examine each of these situations. When two routers first become neighbors, they exchange topology tables during startup mode.