Consider the network in 9.2.1.1 Split Horizon:, using distance-vector routing updates. B and Cs table entries for destination D are shown. All link costs are 1.

Suppose the DA link breaks and then these update reports occur:

A reports D, to B (as before)

C reports D,2 to B (as before)

A now reports D, to C (instead of B reporting D,3 to A)

(a). Give A, B and Cs forwarding-table records for destination D, including the cost, after these three reports.

(b). What additional reports (a pair should suffice) will lead to the formation of the routing loop?

(c). What (single) additional report will eliminate the possibility of the routing loop?

9.2.1 split horizon:

Under split horizon, if A uses N as its next_hop for destination D, then A simply does not report to N that it can reach D; that is, in preparing its report to N it first deletes all entries that have N as next_hop. In the example above, split horizon would mean B would never report to A about the reachability of D because A is Bs next_hop to D.

Split horizon prevents all linear routing loops. However, there are other topologies where it cannot prevent loops.

C (D,A,2) C (D,A,2)