Suppose a set of routers all use the split-horizon technique; we consider here under what circumstances it makes a difference if they use poison reverse in addition.

(a) Show that poison reverse makes no difference in the evolution of the routing loop in the two examples described in Section 3.4.2, given that the hosts involved use split horizon.

(b) Suppose split-horizon routers A and B somehow reach a state in which they forward traffic for a given destination X toward each other. Describe how this situation will evolve with and without the use of poison reverse.

(c) Give a sequence of events that leads A and B to a looped state as in (b), even if poison reverse is used. Hint: Suppose B and A connect through a very slow link. They each reach X through a third node, C, and simultaneously advertise their routes to each other.

3.4.2 Distance-Vector (RIP)

The idea behind the distance-vector algorithm is suggested by its name. (The other common name for this class of algorithm is Bellman–Ford, after its inventors.) Each node constructs a one-dimensional array (a vector) containing the “distances” (costs) to all other nodes and distributes that vector to its immediate neighbors. The starting assumption for distance-vector routing is that each node knows the cost of the link to each of its directly connected neighbors. These costs may be provided when the router is configured by a network manager. A link that is down is assigned an infinite cost.