If we consider input and output port functionality and the configurations shown in Figure 4.8, it’s clear that packet queues may form at both the input ports and the output ports, just as we identified cases where cars may wait at the inputs and outputs of the traffic intersection in our roundabout analogy. The location and extent of queueing (either at the input port queues or the output port queues) will depend on the traffic load, the relative speed of the switching fabric, and the line speed. Let’s now consider these queues in a bit more detail, since as these queues grow large, the router’s memory can eventually be exhausted and packet loss will occur when no memory is available to store arriving packets. Recall that in our earlier discussions, we said that packets were “lost within the network” or “dropped at a router.” It is here, at these queues within a router, where such packets are actually dropped and lost.