Chapter 7 - 7.3 - Communication Protocols - Practice Problems - Page 267: 3

If the link connecting node $\mathrm{F}$ to node $\mathrm{D}$ fails, then the paths ABFD and AEFD will not work and their weights become "infinite." Of the two paths remaining, path ABCD with weight 16 now becomes the shortest path. No one link in the network will disconnect nodes $\mathrm{A}$ and D. We can see that clearly by noting that the two paths $A B C D$ and AEFD do not share any links in common. Therefore, if a link along one of these paths fails, we can use the other path.

If the link connecting node $\mathrm{F}$ to node $\mathrm{D}$ fails, then the paths ABFD and AEFD will not work and their weights become "infinite." Of the two paths remaining, path ABCD with weight 16 now becomes the shortest path. No one link in the network will disconnect nodes $\mathrm{A}$ and D. We can see that clearly by noting that the two paths $A B C D$ and AEFD do not share any links in common. Therefore, if a link along one of these paths fails, we can use the other path.