What Is Network Congestion?
Network Congestion — the condition when a network link or device carries more traffic than it can handle efficiently — causing delay, jitter and packet loss as queues overflow.
How it works
Every link has finite bandwidth. When traffic demand exceeds it, packets queue up in device buffers; if the queue fills, additional packets are dropped. TCP responds to congestion by slowing down (congestion control algorithms), while UDP-based apps simply experience quality degradation.
Why it matters
Congestion is why QoS exists — prioritising critical traffic during congestion instead of treating everything equally. Diagnosing it involves checking interface utilisation and output drops (show interfaces) to find the bottleneck link.
Frequently asked questions
What causes network congestion?
Traffic demand exceeding a link or device's capacity — too many devices, insufficient bandwidth, or a bottleneck somewhere in the path.
What happens during congestion?
Packets queue in buffers; if queues fill, packets are dropped, causing increased latency, jitter and packet loss for real-time applications.
How is congestion managed?
QoS prioritises important traffic, capacity planning increases bandwidth where needed, and TCP's own congestion control algorithms automatically slow senders during congestion.
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