Route Flap and Dampening

Route flapping is when an advertised route is changing back and forth, or a route is advertised, withdrawn, advertised, woithdrawn, etc..  It creates havoc because all the routers that store that route have to keep changing it.  The outside world of routers, to protect themselves, apply a “penalty” (dampen) to anyone who is flapping their route/s, and this causes them to stop honoring those routes for a period of time

If your routes flap more than one or two complete up-down-up cycles, you will be dampened by many providers for at least an hour or so!!

When you "assert" a route - saying "I know how to get to 192.204.4.0/24" and then later "withdraw" that assertion if you in fact no longer know how to get to 192.204.4.0.  But look at what happens when you withdraw that assertion. Your provider(s) must then also withdraw that assertion. And then their provider(s) and peer(s) must do the same. All in all, thousands of routers around the world now have to look at that route and decide if they have a next-best path in their BGP (or other routing) table, and insert it as the current best path in their IP routing table. This consumes many CPU-seconds on routers that are sometimes very busy.

In fact, it was consuming so much CPU time a few years ago that a few people came up with an idea (which Cisco implemented in record time) to "damp"(en) the "route flap"s. You'll hear people say "damp" and "dampen". There's no real consensus about which is the correct term.

What this means in practice today is that if your routes flap more than one or two complete up-down-up cycles, you will be dampened by many providers for at least an hour or so. So even if you're only "single-homed", you will be dampened if your provider withdraws your routes every time your T1 flips up and down a few times because some Bell guy tripped over a wire.

So do not ask your upstream provider to announce you unless it makes a difference (the benefit of being multiply-announced outweighs the possible negative effects of being dampened due to instability in either your or your provider's network).