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Damping Physical Interfaces

SUMMARY You (the network administrator) can configure damping to reduce the advertisement of physical interface transitions between up and down states.

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 interface damping on one end and not configuring interface damping on the other end can result in undesired behavior.

The types of interface damping depend 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, 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.

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 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.

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

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 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

Configure Damping of Shorter Physical Interface Transitions

By default, when an interface changes from up to down or from down to up, this transition is advertised immediately to the hardware and Junos OS. In some situations, you might want to damp interface transitions.

For example, you may want to configure damping on an interface that is connected to an add/drop multiplexer (ADM) or wavelength-division multiplexer (WDM), or to protect against SONET/SDH framer holes.

Damping the interface means not advertising the interface’s transition until a certain period of time has passed, called the hold-time. When 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. If the timer expires and the interface state is still down, then the router begins to advertise the interface as being down. Similarly, when 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. If the timer expires and the interface state is still up, then the router begins to advertise the interface as being up.

To configure damping of shorter physical interface transitions in milliseconds:

  1. Select the interface to damp, where the interface name is interface-type-fpc/pic/port:
  2. Configure the hold time for link up and link down.

The hold time can be a value from 0 through 4,294,967,295 milliseconds. The default value is 0, which means that interface transitions are not damped. Junos OS advertises the transition within 100 milliseconds of the time value you specify.

For most Ethernet interfaces, Junos OS implements hold timers using a one-second polling algorithm. For 1-port, 2-port, and 4-port Gigabit Ethernet interfaces with small form-factor pluggable (SFP) transceivers, hold timers are interrupt driven.

Note:

The hold-time option is not available for controller interfaces.

Configure Damping of Longer Physical Interface Transitions

Physical interface damping limits the advertisement of the up-and-down transitions (flapping) on an interface. An unstable link between a router Interface and the transport devices can lead to periodic flapping. Longer flaps occur with a period of about five seconds or more, with an up-and-down duration of one second.

For these 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, a penalty is added to the interface penalty counter. If at some point the accumulated penalty exceeds the suppress level max-suppress, the interface is placed in the suppress state, and further interface state up-and-down transitions are not reported to the upper-level protocols.

You can view the damping parameters with the show interfaces extensive command.

Note:

Only PTX Series routers, T Series routers, MX2010 routers, MX2020 routers, MX960 routers, MX480 routers, MX240 routers, MX80 routers, and M10i routers support interface damping for longer periodic interface flaps.

To configure damping of longer physical interface transitions:

  1. Select the interface to damp, where the interface name is interface-type-fpc/pic/port or an interface range:
  2. Enable longer interface transition damping on a physical interface:
  3. (Optional) Set the maximum time in seconds that an interface can be suppressed no matter how unstable the interface has been.
    Note:

    Configure max-suppress to a value that is greater than the value of half-life; otherwise, the configuration is rejected.

  4. (Optional) Set the decay half-life in seconds, which is the interval after which the accumulated interface penalty counter is reduced by half if the interface remains stable.
    Note:

    Configure half-life to a value that is less than the value of max-suppress; otherwise, the configuration is rejected.

  5. (Optional) Set the reuse threshold (no units). When the accumulated interface penalty counter falls below this value, the interface is no longer suppressed.
  6. (Optional) Set the suppression threshold (no units). When the accumulated interface penalty counter exceeds this value, the interface is suppressed.
    Note:

    The system does not indicate whether an interface is down because of suppression or because that is the actual state of the physical interface. Therefore, neither SNMP link traps nor Operation, Administration, and Maintenance (OAM) protocols can differentiate the damped version of the link state from the real version. Therefore, 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.

Example: Configure Physical Interface Damping

This example shows how to configure damping for a physical interface on a PTX Series Packet Transport Router.

Requirements

This example uses the following hardware and software components:

  • One PTX Series Packet Transport Router

  • One or more routers that provide input packets and receive output packets

  • Junos OS Release 14.1 or later

Overview

Physical interface damping provides a smoothing 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 these categories:

  • Nearly instantaneous multiple flaps of short duration (milliseconds). 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 interface is not advertised to the rest of the system as being down until it has remained down for the hold-down timer period. Similarly, when a hold-up timer is configured and an interface goes from down to up, it is not advertised as being up until it has remained up for the hold-up timer period.

  • 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 state up transitions are not reported to the upper-level protocols.

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Procedure

Step-by-Step Procedure

To configure damping on the PTX Series Packet Transport Router:

  1. Set the half-life interval, maximum suppression, reuse, suppress values, and enable:

  2. Commit the configuration:

Results

From configuration mode, confirm your configuration by entering the show interfaces command. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Verification

To confirm that the configuration is working properly, perform this task:

Verify Interface Damping on xe-6/0/0

Purpose

Verify that damping is enabled on the interface and that the damping parameter values are correctly set.

Action

From operational mode, run the show interfaces extensive command.

Meaning

Damping is enabled and configured successfully on the xe-6/0/0 interface.