Three years later, you're still consulting for CNT Books. The network has more than 15 subnets and 10 routers in several buildings and locations. You have been keeping up with the network by configuring the routers statically. However, users have had problems with downtime in the past year because of network links going offline, as there's only one route to reach every subnet. The owner wants fault tolerance built into the network to include backup links in case a primary link goes offline. You're concerned that the current router configuration method will still cause some downtime, even if the backup links operate correctly. Why might there be downtime if a primary link goes offline but the backup link is okay