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Cisco 12000 series routers

Find out about the dual-buffered cross-bar switch fabric that Cisco 12000 series routers use.

The Cisco 12000 series routers use a dual-buffered cross-bar switch fabric. This means that packets are buffered both at the input interface and at the output interface. Being a cross-bar switch means that every port can talk to every other port directly via dedicated channels. There is no shared bandwith on the backplane. Another type of switch fabric is bus. With a bus fabric, all ports talk to each other via a shared pipe on the backplane.

Having buffering on both the input ports and the output ports allows the router to avoid blocking. Blocking occurs when multiple input ports try to send data to the same output port and the output port can't keep up with the amount of traffic being sent to it. This happens a lot on FDDI switches when multiple 100-Mbps interfaces try to send data to one 100-Mbps interface. The buffers fill up, and data is lost. Cisco combats this by checking the output port's buffers before sending the data to it. If the output port buffer is full, the data is queued on the input port until room is available on the output port.

Source: Cisco Router Configuration & Troubleshooting by Mark Tripod, New Riders, 1999

To find out more about Cisco Router Configuration & Troubleshooting, go to and
This was last published in August 2000

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