Stacked communication devices

Port mirroring is achieved between ports (e.g. A,E) on separate devices 210, 220, 230 in a stack of communication devices by establishing a protocol for communications within a cascade connection 250 forming the stack in which an indication is given of whether the communication is being sent to the mirror port (e.g. E) in addition to its intended destination (e.g. via ports B, C, D). This avoids increasing the volume of traffic in the cascade connection as would be the case if the original and mirror copy were sent separately. Computing device 240 thus monitors the activity of computing device 248. A roving analysis port (RAP) bit or field is added to the packet for transmission over the cascade connection 250, to indicate whether it needs sending to the monitoring port. The same principle is applicable for copying (diverting) to a security port.