Interfaces and Hardware Component Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

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Interfaces and Hardware Component Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

Carrier delay on physical interfaces

Updated: July 31, 2026

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Describes how the carrier delay introduces a delay in processing interface link-state notifications on physical Ethernet interfaces so that the interface link has time to stabilize.

The carrier delay is a ethernet interfaces configuration that

  • processes interface link-state notifications

  • provides time for the interface link to stabilize, and

  • avoids unnecessary interface state changes.

Hardware links experience link flaps, which are conditions where a physical interface fluctuates between up and down states. These flaps cause the network to reestablish routing paths and lead to resource exhaustion on routers. The system manages link stability through these timer processes:

  • The carrier-delay up timer starts when the link state transitions to an up state.

  • The carrier-delay down timer starts when the link state transitions to a down state.

  • The Ethernet interface maintains its current state during the delay period until the timer expires.

Feature history

Feature name

Release information

Feature description

Default carrier delay value on physical interfaces

Introduced in this release on: Modular Systems (8800 [LC ASIC: K100])(select variants only*)

*This feature is supported on Cisco 88-LC1-48Y8F-EM line cards.

Default carrier delay value on physical interfaces

Release 26.1.1

Introduced in this release on: Centralized Systems (8400 [ASIC:K100]) )(select variants only*)

*This feature is now supported on Cisco 8404-SYS-D routers.

Default carrier delay value on physical interfaces

 Release 25.4.1

Introduced in this release on: Fixed Systems (8010 [ASIC: A100], 8700 [ASIC: K100])(select variants only*)

*This feature is now supported on:

  • 8711-48Z-M

  • 8011-32Y8L2H2FH

  • 8011-12G12X4Y-A/D

Default carrier delay value on physical interfaces

Release 25.1.1

Introduced in this release on: Fixed Systems (8010 [ASIC: A100])(select variants only*)

*This feature is supported on Cisco 8011-4G24Y4H-I routers.

Default carrier delay value on physical interfaces

 Release 24.4.1

Introduced in this release on: Fixed Systems (8200 [ASIC: P100], 8700 [ASIC: P100, K100])(select variants only*); Modular Systems (8800 [LC ASIC: P100])(select variants only*)

You can now set the carrier-delay up default value on the listed hardware that provides sufficient time to establish a stable hardware link state. If you have not set the default timer, the default carrier delay automatically delays the hardware link-up notifications by 200 ms.

*This feature is now supported on:

  • 8212-48FH-M

  • 8711-32FH-M

  • 8712-MOD-M

  • 88-LC1-12TH24FH-E

  • 88-LC1-36EH+A8:B12

  • 88-LC1-52Y8H-EM

Default carrier delay value on physical interfaces

 Release 24.3.1

Introduced in this release on: Fixed Systems(8200, 8700)(select variants only*); Modular Systems (8800 [LC ASIC: P100])(select variants only*).

We have introduced the carrier-delay up default value to ensure enough time to establish a stable hardware link state. If you haven’t configured the timer, the default carrier delay automatically delays the hardware link-up notifications by 200 ms.

Previously, we recommended that you set the carrier delay-up timer to 10 ms.

If you want to change the delay of the interface state change notification, you can use the carrier-delay command to set a different value.

*This feature is now supported on:

  • 8212-48FH-M

  • 8711-32FH-M

  • 88-LC1-12TH24FH-E

  • 88-LC1-52Y8H-EM

Default carrier delay value on physical interfaces

Release 24.2.11

Introduced in this release on: ; Modular Systems (8800 [LC ASIC: P100])(select variants only*).

We have introduced the carrier-delay up default value to ensure enough time to establish a stable hardware link state. If you haven’t configured the timer, the default carrier delay automatically delays the hardware link-up notifications by 200 ms.

Previously, we recommended that you set the carrier delay-up timer to 10 ms.

If you want to change the delay of the interface state change notification, you can use the carrier-delay command to set a different value.

*This feature is now supported on 88-LC1-36EH.

Setting the carrier delay on physical interfaces

 Release 7.5.4

You can configure the Ethernet interfaces to delay the processing of hardware link-down and link-up notifications. With this functionality, the interface state remains stable for the configured delay duration, even if the hardware link state fluctuates. This prevents interface flapping and improves network reliability.

Use the following CLI command in interface configuration mode to configure the delay time:

carrier-delay

Guidelines for carrier delay

These guidelines apply when you configure carrier delay.

  • You can configure carrier-delay for only link-up, only link-down, or both link-up and link-down notifications.

  • From Release 24.2.11, the default value of the carrier-delay up parameter is changed to 200 ms. To restore your original configuration, configure the parameter explicitly by using the carrier-delay command.

  • If not configured, the carrier-delay up parameter defaults to 200 ms and the carrier-delay down parameter defaults to 0 ms. When carrier-delay down is not configured, the higher-layer protocols are notified immediately when a physical link state changes.

  • The carrier-delay command overwrites the previous configuration every time you execute the command. For example, if you already have the carrier-delay up configured and later configure the carrier-delay down , the carrier-delay down overwrites the previously configured carrier-delay up. However, you can configure the carrier delay up and down concurrently by using the carrier-delay up (milliseconds) down (milliseconds) command.

Restrictions of carrier delay

These restrictions apply when you configure carrier delay:

  • If the carrier-delay down milliseconds command is configured on a physical link that fails and cannot be recovered, link-down detection time increases, and it may take longer for the routing protocols to reroute the traffic around the failed link.

  • Loss of Signal (LOS) is not supported on carrier delay.

Configure the carrier-delay timer

This example shows that when you bring up an interface without configuring the carrier-delay command, the carrier-delay configuration for the interface defaults to 200 ms:


Router#configure
Router(config)#interface HundredGigE 0/0/0/0
Router(config-if)#no shutdown
Router(config-if)#commit

Run the show interfaces command to check the default carrier-delay value:


Router#show interfaces HundredGigE 0/0/0/0 | include Carrier
Fri Mar 31 07:25:05.273 UTC
Carrier delay (up) is 200 msec

To change the carrier-delay up and carrier-delay down values on an interface, perform the following steps. In this example, link-up notifications are delayed by 1000 ms and link-down notifications are delayed by 150 ms.

Procedure

1.

Enter global configuration mode.

Example:

Router#configure
2.

Enter interface configuration mode for the target physical interface.

Example:

Router(config)#interface HundredGigE 0/0/0/0
3.

Configure the carrier-delay up and carrier-delay down values on the interface.

Example:

Router(config-if)#carrier-delay up 1000 down 150
4.

Commit the configuration, then verify the running configuration and the carrier-delay values on the interface.

Example:

Router(config-if)#commit

The following running configuration shows the carrier-delay up and carrier-delay down values on interface HundredGigE0/0/0/0:


 interface HundredGigE0/0/0/0
 carrier-delay up 1000 down 150
!
5.

Run the show interfaces command to see the current state of the carrier-delay configuration for the interface.

Example:


Router#show interfaces HundredGigE 0/0/0/0 | include Carrier
Fri Mar 31 07:25:05.273 UTC
Carrier delay (up) is 1000 msec, Carrier delay (down) is 150 msec