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Physical Interface Damping Overview

 

Physical interface damping limits the advertisement of the up and down transitions (flapping) on an interface. Each time a transition occurs, the interface state is changed, which generates an advertisement to the upper-level routing protocols. Damping helps reduce the number of these advertisements.

From the viewpoint of network deployment, physical interface flaps fall into the following categories:

  • Nearly instantaneous multiple flaps of short duration (milliseconds).

  • Periodic flaps of long duration (seconds).

Figure 1 is used to describe these types of interface flaps and the damping configuration that you can use in each case.

Figure 1: Two Router Interfaces Connected Through Transport Equipment
Two Router Interfaces Connected Through
Transport Equipment
Note

We recommend that you use similar damping configurations on both ends of the physical interface. Configuring damping on one end and not having interface damping on the other end can result in undesired behavior.

The following sections describe the types of interface damping depending upon the transition time length.

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, link switching is performed. Link switching takes a number of milliseconds. As shown in Figure 2, during switching, both router 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.

Figure 2: Multiple Flaps of Short Duration (Milliseconds)
Multiple Flaps of Short Duration (Milliseconds)

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 as 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, so fails to suppress periodically flapping interfaces on the system.

Figure 3: Periodic Flaps of Long Duration (Seconds)
Periodic Flaps of Long Duration (Seconds)

For longer periodic interface flaps, you 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, a penalty is added to the interface penalty counter. If at some point the accumulated penalty exceeds the suppress level, the interface is placed in the suppress state, and further interface link up and down events are not reported to the upper-level protocols.

Note
  • Only PTX Series routers, T Series routers, MX960 routers, MX480 routers, MX240 routers, MX80 routers, and M10i routers support interface damping for longer periodic interface flaps on all the line cards.

  • Penalty added on every interface flap is 1000.

  • The system does not indicate whether an interface is down because of suppression or that is the actual state of the physical interface. Because of this, SNMP link traps and Operation, Administration, and Maintenance (OAM) protocols cannot differentiate the damped version of the link state from the real version. Therefore, the traps and protocols might not work as expected.

  • You can verify suppression by viewing the information in the Damping field of the show interface extensive command output.

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 4: Physical-Level Link Is Down When the Penalty Falls Below the Reuse Level
Physical-Level Link Is Down When the
Penalty Falls Below the Reuse Level

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.

Figure 5: Physical-Level Link Is Up When the Penalty Falls Below the Reuse Level
Physical-Level Link Is Up When the Penalty
Falls Below the Reuse Level