Force10 Link Dampening

First of all let's explain why we should use Link Dampening?

Interface state changes occur when interfaces are administratively brought up or down or if an interface state changes. Every time an interface changes a state or flaps, routing protocols are notified of the status of the routes that are affected by the change in state. These protocols go through the momentous task of re-converging. Flapping; therefore, puts the status of entire network at risk of transient loops and black holes. Link dampening minimizes the risk created by flapping by imposing a penalty for each interface flap and decaying the penalty exponentially. After the penalty exceeds a certain threshold, the interface is put in an Error-Disabled state and for all practical purposes of routing, the interface is deemed to be “down.” After the interface becomes stable and the penalty decays below a certain threshold, the interface comes up again and the routing protocols re-converge.

Dampening parameters:

Syntax: dampening [[[[half-life] [reuse-threshold]] [suppress-threshold]] [max-suppress-time]]

·         half-life

o    The number of seconds after which the penalty is decreased. The penalty decreases half after the half-life period expires. The range is from 1 to 30 seconds. The default is 5 seconds.

·         reuse-threshold

o    The number as the reuse threshold, the penalty value below which the interface state is changed to “up”. The range is from 1 to 20000. The default is 750.

·         suppress-threshold

o    The number as the suppress threshold, the penalty value above which the interface state is changed to “error disabled”. The range is from 1 to 20000. The default is 2500.

·         max-suppress-time

o    The maximum number for which a route can be suppressed. The default is four times the half-life value. The range is from 1 to 86400. The default is 20 seconds.

Dampening algorithm:

With each flap, Dell Networking OS penalizes the interface by assigning a penalty (1024) that decays exponentially depending on the configured half-life. After the accumulated penalty exceeds the suppress threshold value, the interface moves to the Error-Disabled state. This interface state is deemed as “down” by all static/dynamic Layer 2 and Layer 3 protocols. The penalty is exponentially decayed based on the half-life timer. After the penalty decays below the reuse threshold, the interface enables.

Dampening settings timing example: 

Lets say we have dampening 10 100 1000 60

·         half-life = 10 seconds

·         reuse-threshold = 10

·         suppress-threshold = 1000

·         max-suppress-time = 60 second

Time after flap	Penalty	Port state	Comment
0s	1024	Down	Penalty set to 1024 Penalty (1024) > Supress-threshold (1000)  then port state down
10s	512	Down	Penalty set to 1024 / 2 Penalty (512) > Reuse-threshold (100) then port state still down
20s	256	Down	Penalty set to 512 / 2 Penalty (256) > Reuse-threshold (100) then port state still down
30s	128	Down	Penalty set to 256 / 2 Penalty (256) > Reuse-threshold (100) then port state still down
40s	64	Up	Penalty set to 128 / 2 Penalty (64) < Reuse-threshold (100) then port state is changed to UP