A. Only routers can split boundaries (horizons) between concentric networks.
B. Each AS must keep routing tables converged to prevent dead routes from being advertised across boundaries.
C. Networks can only remain fully converged if all information is sent out all active interfaces.
D. Information about a route should not be sent back in the direction from which the original update came.
E. Distance vector protocols need fall back routers that are responsible for momentary loops.