Hold down is another distance-vector loop avoidance technique, whereby hosts ignore updates for a period of time until link failure news has had a chance to propagate. Consider the networks in Figure 3.55, where all links have cost 1 except E–D with cost 10.

Suppose that the E–A link breaks and B reports its loop-forming E route to A immediately afterwards (this is the false route, via A).

Specify the details of a hold-down interpretation, and use this to describe the evolution of the routing loop in both networks. To what extent can hold-down prevent the loop in the EAB network without delaying the discovery of the alternative route in the EABD network?

Figure 3.55)

(E) (A) B (D and (E) (A) B