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

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

Configure host side output loopback

Updated: June 16, 2026

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Provides instructions on how to configure host side output loopback in a controlled workflow. Covers prerequisites, execution steps, and result checks needed for successful completion.

In host loopback line or host side output loopback, the signals received at the host side are looped back to the host side, indicating that the received data on the host is transmitted back to the host, that is, towards the local interface. This is indicated by the arrows labeled as 1 and 2 in the illustration.

Figure 1. Host Side Output Loopback on the Optical Transceiver
Host Side Output Loopback on the Optical Transceiver

Procedure

  1. Configure host side output loopback.

    Example:

    Router#config
Router(config)#controller optics 0/0/0/9
Router(config-Optics)#secondary-admin-state maintenance
Router(config-Optics)#host loopback internal
Loopback host is a traffic-affecting operation
Router(config-Optics)#commit
Router(config-Optics)#end

  2. Display the running configuration for host side output loopback.

    Example:

    Router#show run controller optics 0/0/0/9
controller Optics0/0/0/9
 secondary-admin-state maintenance
 host loopback internal
!

  3. Verify host side output loopback.

    Example:

    Router#show controller optics 0/0/0/9
Controller State: Up
Transport Admin State: Maintenance
Laser State: On
LED State: Green
FEC State: FEC ENABLED
Optics Status:

Optics Type: QSFPDD 400G FR4
Wavelength: 1301.00 nm
Loopback Host: Internal
Loopback Media: None

Alarm Status:
----------------
Detected Alarms: None
LOS/LOL/Fault Status:
Performance Monitoring: Disable