What Is Packet Loss?
Packet Loss — the percentage of packets sent that never arrive at their destination — caused by congestion, faulty hardware, wireless interference or overloaded links.
How it works
When a link is congested, routers/switches drop packets they can't queue (buffer overflow) rather than delay everything. TCP notices missing packets (via missing acknowledgments) and retransmits them — trading a little delay for reliability. UDP-based applications (voice, video, gaming) don't retransmit, so packet loss shows up directly as glitches or gaps.
Why it matters
Even small packet loss (1-2%) is noticeable in real-time applications, while file transfers barely feel it thanks to TCP retransmission. Diagnosing packet loss usually starts with extended ping tests and checking interface error counters for drops.
Frequently asked questions
What causes packet loss?
Network congestion (buffer overflow), faulty cabling or hardware, wireless interference, or an overloaded device dropping packets it can't process in time.
How much packet loss is acceptable?
For file transfers, TCP handles even a few percent gracefully via retransmission. For voice/video, anything above 1-2% becomes noticeably disruptive.
How do you test for packet loss?
Extended ping with a high repeat count measures loss percentage directly; interface counters (show interfaces) reveal drops at a specific device.
Related articles
Want hands-on training?
Learn this on real Cisco lab devices with placement support at Attila Technologies, Ahmedabad.