Suppose we have switches S1 through S4; the forwarding-table destinations are the switches themselves. The tables for S2 and S3 are as below, where the next_hop value is specified in neighbor form: S2: S1,S1 S3,S3 S4,S3 S3: S1,S2 S2,S2 S4,S4 From the above we can conclude that S2 must be directly connected to both S1 and S3 as its table lists them as next_hops; similarly, S3 must be directly connected to S2 and S4.

(a). The given tables are consistent with the network diagrammed below. Are the tables also consistent with a network in which S1 and S4 are not directly connected? If so, give such a network; if not, explain why S1 and S4 must be connected.

(b). Now suppose S3s table is changed to the following. Find a network layout consistent with these tables in which S1 and S4 are not directly connected. S3: S1,S4 S2,S2 S4,S4 While the table for S4 is not given, you may assume that forwarding does work correctly. However, you should not assume that paths are the shortest possible; in particular, you should not assume that each switch will always reach its directly connected neighbors by using the direct connection.

S1S2     S4S3