An Ethernet link that repeatedly changes from up to down is called a link flap or port flap. The symptom can look like random packet loss, short disconnects, dropped calls, unstable remote sessions, or a device that disappears for a few seconds and then comes back.

The first place to look is usually Layer 1. Cisco’s Catalyst port-flap troubleshooting guide lists bad or loose cabling, faulty or unsupported transceivers, bad switch ports, negotiation problems, and endpoint behavior among the common causes. A switch log that shows repeated interface-up and interface-down events is one of the clearest signs that the physical link itself is unstable.

Start With the Physical Path

Reseat both ends of the cable, then replace the patch cable with a known-good one. If the path crosses a wall jack or patch panel, bypass intermediate connections when practical. For fiber, reseat or replace the optic and patch cord, verify the transceiver is supported, and inspect optical power levels when the hardware exposes digital optical monitoring.

This is closely related to the troubleshooting workflow in Why Gigabit Ethernet Falls Back to 100 Mbps. A marginal copper pair can sometimes keep a link alive at a lower speed before the fault becomes severe enough to make the link flap entirely.

Link flapping intermittently
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Check What the NIC and Switch See

On Linux, ethtool can show negotiated speed, duplex, link state, driver information, and hardware counters. On managed switches, check the interface log and counters for frequent state transitions, CRC errors, symbol errors, drops, or transceiver alarms. The earlier Ethernet link-negotiation guide explains how speed, duplex, auto-negotiation, LEDs, and interface errors fit together.

A cable tester can shorten the search dramatically. Fluke Networks’ LinkIQ documentation describes tests for wire map, opens, shorts, split pairs, distance to fault, cable bandwidth, advertised switch speed, duplex, and port information. Those measurements help distinguish a physical cabling problem from a configuration or endpoint problem.

Do Not Confuse Link Flapping With MAC Flapping

They sound similar but describe different failures. Link flapping means the physical interface itself is repeatedly losing and regaining carrier. MAC flapping means a switch keeps learning the same MAC address on different ports, which can point to loops, topology changes, virtualization behavior, or another Layer 2 problem.

The practical troubleshooting order is simple: confirm the up/down events in logs, replace or reseat the physical components, test the cable or optics, inspect counters, then investigate negotiation, drivers, power-saving behavior, LACP, or other protocol-level causes. Changing settings before ruling out the physical path often makes an intermittent fault harder to isolate.

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