My moel B is flashing all three network lights (FDX, LNK and 100) toghether about once per second. Is this an error message? There is a cable plugged in but there appears to be a break somewhere between Pi and switch.
To clarify, I have found the actual problem - broken cabling somewhere between the wall socket and the switch - but I was puzzled by the behaviour of the indicators and I was wondering if I had missed some diagnostic that would have helped.
This behavior is typical of an Ethernet interface that detects some signal on a cable, and is trying to establish a link but fails to do so due to either excessive noise, too long of a cable, a broken wire or missing pair, or failure of auto-negotiation (rare) due to hardware incompatibility. It then repeats the process after the timeout as the degraded signal is still there.
It cannot be relied upon because it is specific to the PHY chip and the low level configuration details, as well as the PU/PD state of the LEDs. But in my experience this is a typical indication. To be clear this is usually indicative of a Layer 1 (Physical Interface) Link Failure and retry.
Link detection and link negotiation are complicated topics, and what happens at the lowest level is often specific to the PHY itself. But generally speaking the chip will detect a signal present and try to negotiate a link. If one pair of the TX/RX is broken, or the signal is degraded due to one wire in the pair being broken, the detection circuit may trigger (it sees some signal), but establishing the link is unsuccessful, during this brief period of time the LED's may be driven to a default state that can cause the LED's to light.
In the case of gigabit ethernet, a missing pair should fall-back to 100Mbit, but occasionally the MDI/MDI-X fails due to signal degradation and the two sides cannot agree on a link.
The Ethernet lights indicate the presence of traffic and if the port is operating at top speed. They show a working connection and if data is being transmitted.
