A crossbar switch architecture cross connects each port to all the other ports via the backplane. This means that the switch requires N x N connections, adding to the complexity of the switch. Because there is a direct connection between all ports, traffic can be forwarded directly to multiple ports simultaneously. All ports receive a copy of the inbound frame, but not all ports are permitted to transmit the frame to the wire. A complex bus arbitration algorithm is needed to make this architecture work.

However a crossbar architecture has a small problem. When a crossbar switch serves multiple networks, and two frames enter the switch at the same time destined for different ports, one of the frames is blocked while the first frame is forwarded. This results in all frames being queued as they flow through the switch. If there is sufficient traffic and insufficient buffer space on the switch, packets are dropped.

This problem is called Head of Line Blocking, and is a common problem with crossbar switches. One device that suffers from just such a problem is the old DEC gigaswitch design.

Compare this to the hierarchical switch design.


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