Oversubscribing the Switch Fabric

Switch manufacturers use the term non-blocking to indicate that some or all the switched ports have connections to the switch fabric equal to their line speed. For example, an 8-port Gigabit Ethernet module would require 8 Gb of bandwidth into the switch fabric for the ports to be considered non-blocking. All but the highest end switching platforms and configurations have the potential of oversubscribing access to the switching fabric.

Depending on the application, oversubscribing ports may or may not be an issue. For example, a 10/100/1000 48-port Gigabit Ethernet module with all ports running at 1 Gbps would require 48 Gbps of bandwidth into the switch fabric. If many or all ports were connected to high-speed file servers capable of generating consistent streams of traffic, this one-line module could outstrip the bandwidth of the entire switching fabric. If the module is connected entirely to end-user workstations with lower bandwidth requirements, a card that oversubscribes the switch fabric may not significantly impact performance.

Cisco offers both non-blocking and blocking configurations on various platforms, depending on bandwidth requirements. Check the specifications of each platform and the available line cards to determine the aggregate bandwidth of the connection into the switch fabric.