the switching fabric

The switching fabric is the internal plumbing of a router or switch that physically moves a packet from the input port where it arrived to the output port it must leave by. Once the forwarding table has decided the exit, the fabric is the machinery that actually carries the packet across the box. Imagine a busy mailroom: the lookup decides which outgoing bin a parcel belongs in, and the conveyor belts and chutes that whisk it to that bin are the fabric.

There are three classic designs. Switching via memory is the simplest and oldest: the packet is copied into the router's main memory and then out to the chosen port, limited by memory bandwidth (every packet crosses the memory bus twice). Switching via a bus sends packets across one shared internal bus, simple but a bottleneck because only one packet can cross at a time. Switching via an interconnection network (a crossbar) is the modern high-end answer: a grid of switch points lets many input-output pairs transfer simultaneously, so the device can run at full line rate. Real high-speed routers use crossbars, often multistage.

The fabric is where speed limits and queuing live. If packets arrive faster than the fabric or the output link can drain them, they must wait in queues, which is where queuing delay and packet loss (from buffer overflow) come from inside a router. A notorious effect is head-of-line blocking at input ports: a packet stuck waiting for a busy output can block the packets behind it even when their outputs are free — which is why fabric and buffer design is a serious part of building fast routers.