Interface Counters Explained: Reading Errors for the Network+ Exam
Interface statistics are where physical-layer problems become visible. The Network+ exam expects you to interpret them, because each counter points at a different cause. Interface and line protocol state The first line tells you most of what you need, and the two halves mean different things. Up / up — the interface is working. Up / down — layer 1 is fine but layer 2 is not. Usually an encapsulation mismatch, a keepalive problem, or a mismatch in protocol configuration between the ends. Down / down — no signal. Cable, far-end device powered off, wrong port, or a speed/duplex negotiation failure severe enough to prevent link. Administratively down / down — the port is shut down in configuration. The fix is configuration, not cabling, and it is the first thing to check before anyone goes looking for a cable. The error counters Counter Meaning Usual cause CRC errors Frame checksum failed Cabling, interference, bad optics Runts Frame under 64 bytes Collisions, duplex mismatc...