Damping Interfaces

Damping Overview for Shorter Physical Interface Transitions

Figure 1 shows two routers with two transport devices between them. If a redundant link between the two transport devices fails, Junos OS performs link switching. Link switching takes a number of milliseconds. As shown in Figure 2, during switching, both device interfaces might encounter multiple flaps with an up-and-down duration of several milliseconds. These multiple flaps, if advertised to the upper-level routing protocols, might result in undesired route updates. This is why you might want to damp these interface flaps.

Note:

Damping is suitable only with routing protocols.

For shorter physical interface transitions, you configure interface damping with the hold-time statement on the interface. The hold timer enables interface damping by not advertising interface transitions until the hold timer duration has passed. When a hold-down timer is configured and the interface goes from up to down, the down hold-time timer is triggered. Every interface transition that occurs during the hold time is ignored. When the timer expires and the interface state is still down, then the router begins to advertise the interface as being down. Similarly, when a hold-up timer is configured and an interface goes from down to up, the up hold-time timer is triggered. Every interface transition that occurs during the hold time is ignored. When the timer expires and the interface state is still up, then the router begins to advertise the interface as being up.

Damping Overview for Longer Physical Interface Transitions

When the link between a router interface and the transport devices is not stable, this can lead to periodic flapping, as shown in Figure 3. Flaps occur in the order of seconds or more, with an up-and-down flap duration in the order of a second or more. In this case, using the hold timer feature might not produce optimal results because it cannot suppress the relatively longer and repeated interface flaps. Increasing the hold-time duration to seconds still allows the system to send route updates on the flapping interface. Increasing the duration therefore fails to suppress periodically flapping interfaces on the system.

For longer periodic interface flaps, configure interface damping with the damping statement on the interface. This damping method uses an exponential back-off algorithm to suppress interface up-and-down event reporting to the upper-level protocols. Every time an interface goes down, Junos OS adds a penalty to the interface penalty counter. If at some point the accumulated penalty exceeds the suppress level, Junos OS places the interface in the suppress state. In this case, Junos OS does not report further interface link up-and-down events to the upper-level protocols.

The penalty added on every interface flap is 1000. At all times, the interface penalty counter follows an exponential decay process. Figure 4 and Figure 5 show the decay process as it applies to recovery when the physical level link is down or up. As soon as the accumulated penalty reaches the lower boundary of the reuse level, the interface is marked as unsuppressed, and further changes in the interface link state are again reported to the upper-level protocols. You use the max-suppress option to configure the maximum time for restricting the accumulation of the penalty beyond the value of the maximum penalty. The value of the maximum penalty is calculated by the software. The maximum penalty corresponds to the time it would take max-suppress to decay and reach the reuse level. The penalty continues to decay after crossing the reuse level.

Figure 4 and Figure 5 show the accumulated penalty and the decay over time as a curve. Whenever the penalty is below the reuse level and the physical level link changes state, state changes are advertised to the system and cause SNMP state changes.

Figure 4 shows the penalty dropping below the reuse level when the physical link is down. The system is notified of a state change only after the physical level link transitions to up.

Figure 5 shows the penalty dropping below the reuse level when the physical link is up. The system is notified of a state change immediately.