Route flapping is a route repeatedly appearing, disappearing and reappearing. It’s a classic cause of the most frustrating kind of network problem: the one that’s “slow sometimes” and never quite reproducible.
What it is
When a route flaps, routers must repeatedly recompute and reconverge. Each cycle wastes resources and can cause brief packet loss. A single flap is harmless; chronic, low-grade flapping is corrosive — it degrades performance intermittently without ever triggering a hard “down” alarm.
Because it comes and goes, flapping is notoriously hard to catch. By the time someone logs in to investigate, the route looks fine again.
Why it matters
Intermittent instability generates a steady drip of user complaints (“the app is slow,” “calls drop occasionally”) that are almost impossible to pin down without the right data. The root cause might be a failing link, a flapping neighbour, or a timer mismatch — but you can’t fix what you can’t see, and a point-in-time view never captures a problem that’s defined by coming and going.
What good looks like
Detecting flapping requires watching routing state over time, not just right now — spotting routes that toggle, measuring how often and on what rhythm they do it, and distinguishing a one-off from a periodic oscillation. That’s what turns “it’s slow sometimes” into a specific, fixable fault with a clear pattern behind it.