Ethernet and ARP

Interface counters

Also called show interfaces, ip -s link, input errors, CRC, runts, giants, output drops, input queue drops, carrier transitions, flap, txload

What a card has counted since it started: packets and bytes each way, and every frame it dropped or found damaged.

IOS shows them in show interfaces, Linux in ip -s -s link. CRC errors are frames that arrived damaged (the FCS did not match): a bad cable, noise, or a duplex mismatch. Runts are frames shorter than 64 bytes, giants longer than the MTU allows. Input errors add these up. Output drops are packets the queue in front of the card had no room for: the link is full. Carrier transitions count the link going down and up, a flap. A rate is two readings of the byte counters over time. The counters only grow, so read them twice and compare (or clear counters on IOS first).

See it on real machines

Learn it on real machines.