Summary: [...] A Gigabit Switched Router

McKeown's paper provides a good introduction to the concept of crossbar switches as a departure from the shared backplane, and is invaluable in making sense of the Scaling Routers paper.  In a shared backplane, all ports must share the common bandwidth, and hence large scale backplanes suffer from high cost and low scaleability.  A crossbar eliminates the problem by replacing the shared pipe with a fabric comprising all possible point-to-point links between source and destination port, with switches available to a central scheduler to connect the endpoints.  This design naturally lends itself to efficient one-timestep multicast, as a single source can be crossed with multiple destinations in the switching fabric.  

To achieve 100% throughput, however, the scheduler must ensure that at each timestep, each input is connected to (and passing a cell to) each output, which seems achievable but tricky to predict or guarantee.   This problem is addressed in part by virtual output queueing, in which each input maintains a separate FIFO queue for each output.  In this way, a routable cell will never suffer from head-of-line blocking because it sits in a queue behind packets destined for a congested output link.  What happens when the demand for output ports is not evenly distributed, though?