Course contents

Module 4: BGP PolicyLesson 4.8 (8 of 8 in this module)30 of 43 in the BGP course

Route flap dampening

One unstable link can make a prefix appear and vanish many times a minute, and every router in the internet has to process each change. Dampening is the brake, and it can also keep a good route out for an hour.

Advanced · 12 min read

You'll learn to explain how the penalty, half-life, suppress limit and reuse limit interact, configure and verify dampening, and say when not to use it.

The problem dampening solves

A flap is a route that is withdrawn and then advertised again. A bad cable or a router rebooting in a loop can do this repeatedly. Each flap triggers an UPDATE to every neighbour, which recalculates best paths and passes it on. Across a network the size of the internet that churn costs real CPU time and slows convergence for everyone.

The idea in four steps
The idea in four stepsDampening adds a memory of recent instability.FlapRoute withdrawnthen returnedPenalty+1000 eachflapSuppressOver 2000:route not usedReuseBelow 750:used again

On small screens, swipe sideways or focus the diagram and use the arrow keys.

Penalty is kept per prefix and per neighbour. The route is not removed from the table of the neighbour, it just stops being used.

Penalty, half-life and the two limits

Each time a route flaps, the router adds a penalty to it. The penalty is not fixed: it halves every half-life while the route is stable. Two thresholds turn the number into behaviour.

The parameters and their IOS XE defaults
The parameters and their IOS XE defaultsFive values control dampening.Penalty per flap: 1000Added at each withdrawal.An attribute change adds 500.Half-life: 15 minTime for the penalty to halvewhile the route is stableSuppress limit: 2000Above this, the route issuppressedReuse limit: 750Below this, a suppressedroute is used againMax suppress: 60 minA route never waits longerthan this

On small screens, swipe sideways or focus the diagram and use the arrow keys.

The command takes four of them in order: half-life, reuse, suppress, maximum suppress time.
Route flap dampening: penalty against time
Route flap dampening: penalty against timeA route flaps at minutes 5, 10 and 15 and gains 1000 penalty each time. The penalty halves every 15 minutes. It crosses the suppress limit of 2000 at the third flap, so the route is suppressed until the penalty decays below the reuse limit of 750, at about minute 40.suppress limit 2000reuse limit 750flapflapflap0 min30 min60 min90 min0100020003000route suppressedpenalty

On small screens, swipe sideways or focus the diagram and use the arrow keys.

Defaults on IOS XE: penalty 1000 per flap, half-life 15 minutes, suppress limit 2000, reuse limit 750, maximum suppress time 60 minutes. The figures are computed from those defaults for this example.

Read the curve left to right. Two flaps in the first ten minutes leave the penalty under 2000, so the route is still used. The third flap at minute 15 pushes it just over the suppress limit and the route is suppressed. It stays suppressed while the penalty halves every 15 minutes, and returns when it falls under 750, about 25 minutes later.

Configuring dampening

router bgp 64500 address-family ipv4 unicast bgp dampening

R1. Enables dampening with the defaults: half-life 15, reuse 750, suppress 2000, maximum suppress 60.

router bgp 64500 address-family ipv4 unicast bgp dampening 10 1000 3000 40

R1. Custom values in the order half-life (minutes), reuse, suppress, maximum suppress time (minutes). Here the route is suppressed less easily and for a shorter time.

What counts as a flap
What counts as a flapTwo events add penalty, one does not.Adds penaltyWithdrawal: 1000Attribute change: 500Does notRoute staying upLearned from iBGPvs

On small screens, swipe sideways or focus the diagram and use the arrow keys.

Only eBGP routes are dampened by default.

Verifying dampening

Illustrative Cisco IOS XE output written for this lesson, not a router capture
R1#show ip bgp dampening dampened-paths
BGP table version is 12, local router ID is 192.0.2.254
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete

     Network          From             Reuse    Path
*d 203.0.113.0/24   192.0.2.1        00:25:20 64501 i
d marks a damped route. The Reuse column is the time left until the penalty falls to the reuse limit. The route is in the table but is not selected as best while it is damped.
Illustrative Cisco IOS XE output written for this lesson, not a router capture
R1#show ip bgp dampening flap-statistics
     Network          From             Flaps Duration Reuse    Path
*d 203.0.113.0/24   192.0.2.1            3 00:15:42 00:25:20 64501 i
 h 198.51.100.0/24  192.0.2.1            1 00:02:10          64501 i
Flaps shows how many times each route flapped. h is a history entry: the route is currently withdrawn but R1 remembers its penalty.
clear ip bgp dampening clear ip bgp dampening 203.0.113.0 255.255.255.0

Privileged EXEC mode. Clears the penalty and unsuppresses routes, for all prefixes or for one. Use it after you have fixed the cause of the flap.

Why it is used sparingly today

When one real failure happens, the rest of the internet sees a burst of path changes while it searches for alternatives. The same route can be seen to change several times per router even though its origin flapped once. With aggressive settings, those changes alone push a healthy route over the suppress limit, and a stable prefix disappears from part of the internet for up to an hour. That is the opposite of what you wanted.

RFC 7196 revises the recommended values to far less aggressive ones, and operator guidance such as RIPE-580 recommends against dampening for most networks. If you need to limit instability, fix the cause, and use BFD to detect real failures quickly instead of waiting for the hold timer.

Technical reference: RFC 2439, BGP Route Flap Damping and RFC 7196, Making Route Flap Damping Usable. Output above is illustrative.

Check your understanding

Predict · scenario 1

A route has a penalty of 1600 and has been stable for 15 minutes (half-life 15). What is its penalty now?

Predict · scenario 2

A route is suppressed. Under what condition is it used again?

Predict · scenario 3

Does default dampening on IOS XE suppress a flapping route learned from an iBGP neighbour?

FAQ

Does dampening apply to iBGP routes?
No. By default IOS XE dampens routes learned from eBGP neighbours only. Suppressing iBGP routes could break reachability inside one AS.
How long can a route stay suppressed?
At most the maximum suppress time, 60 minutes by default (four half-lives). The penalty is capped so a route never waits longer than that.
Should I enable dampening on my edge routers?
Think carefully. Current operational guidance (RFC 7196, RIPE-580) warns that aggressive dampening hurts reachability, because a normal burst of changes after one failure can suppress a healthy route. Use it with relaxed values or not at all.