Dell Configuration Guide for the S4048–ON System 9.14.2.5

Important Points to Remember

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. Dampening limits the notification of status to the routing protocols. Link dampening minimizes the risk created by flapping by imposing a penalty (1024) for each interface flap and decaying the penalty exponentially based on the half-time. When the accumulated penalty exceeds a certain threshold (suppress 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 (reuse threshold), the interface comes up again and the routing protocols re-converge.

You configure link dampening using the dampening [[[[half-life] [reuse-threshold]] [suppress-threshold]] [max-suppress-time]] command on the interface.

Following is the detailed explanation of interface state change events:

• suppress-threshold— The suppress threshold is a value that triggers a flapping interface to dampen. The system adds penalty when the interface state goes up and down. When the accumulated penalty reaches the default or configured suppress threshold, the interface state changes to Error-Disabled state. The range of suppress threshold is from 1 to 20000. The default is 2500.

• half-life— The accumulated penalty decays exponentially based on the half-life period. The accumulated penalty decreases half after each half-life period. The range of half-life is from 1 to 30 seconds. The default is 5 seconds.

• reuse-threshold— After exponential decay, the penalty reaches the default or configured reuse threshold. The interface is unsuppressed and the state changes to “up”. The range of reuse threshold is from 1 to 20000. The default is 750.

• max-suppress-time— The maximum amount of time during which the interface remain suppressed. The range is from 1 to 86400. The default is 20 seconds or four times the default half-life period (5 seconds).

• Link dampening is not supported on VLAN interfaces.
• Link dampening is disabled when the interface is configured for port monitoring.
• You can apply link dampening to Layer 2 and Layer 3 interfaces.
• You can configure link dampening on individual interfaces in a LAG.

Configuration Example of Link Dampening

Consider an interface periodically flaps as shown above. Every time the interface goes down, a penalty (1024) is added. In the above example, during the first interface flap (flap 1), the penalty is added to 1024. And, the accumulated penalty will exponentially decay based on the set half-life, which is set as 10 seconds in the above example. During the second interface flap (flap 2), again the penalty (1024) is accumulated. When the accumulated penalty exceeds the configured suppress threshold (2400), the interface state is set to Error-Disabled state. After the flap (flap 3), the interface flap stops. Then, the accumulated penalty decays exponentially and when it reaches below the set reuse threshold (300), the interface is unsuppressed and the interface state changes to “up” state.