How to use the Service Test Portal – Loopback?

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How to use the Service Test Portal – Loopback?


By Clicking on the View and Apply button of the Loopback function, this provides the ability to apply “soft” loopbacks on the Colt Network Interface Devices (NID) at either end of the services

The loopback can be customised depending on the functionality supported by the selected NID and can be applied at Layer 2 or LAyer 3, enabling full end to end tests to be carried out when a test is applied at either end







The following loopback configurations are possible based on the specific service and associated NID selected.*

Layer 2 - Full Service  Layer 2 - Destination or Source MAC Address Layer 3 - Destination or Source IP Address 
All layer 2 Ethernet frames are looped at the designated service end point with destination and source MAC addresses being swapped in the header. Layer 2 Ethernet frames with a designated source or destination MAC address are looped at the service end point.  Layer 3 IP frames with a designated source or destination IP address are looped at the service end point.  
This capability has the potential of looping ALL traffic at the given NID and must be executed with caution. Specifying a source or destination MAC address ensures that other traffic will pass uninhibited. Specifying a source or destination IP address ensures that other traffic will pass uninhibited. 
Note: This functionality is only available on those NIDs that do not support customised Layer 2 loopbacks.  Note: Layer 3 address swapping is disabled when Layer 2 loops are invoked. Note: When Layer 3 loops are invoked, the IP source and destination addresses within the packet are swapped. In addition any TCP/UDP port addresses are also swapped. 

Please be aware that all loopbacks can be set for a defined duration. However all loopbacks will be automatically cleared after 24 hours.

Loopback Capability

ToolFull ServiceLayer 2*Layer 3**
*Layer 2 - Destination of Source MAC Address
**Layer 3 - Destination of Source IP Address
Using the Loopback feature

Select the service and click on View and Apply under the Loopback icon:

Select which end the loopback should be applied, followed by the loop configuration type (Layer 2-Full, Layer 2 or Layer 3 loopback):

When there is no active loopback applied to the service, a message will state that there is no loopback detected.

Click Continue to confirm that you wish to proceed:

Enter the Loop duration and source/destination MAC/IP address. The loop duration can be entered manually in minutes. The maximum is 1,440 minutes (24 hours).

Once complete, click on the Execute Loopback button.


Layer 2 – Full Service Loopbacks is disruptive to your service and will cause loss of traffic

When there is an active loopback applied, a message will be displayed with details about the Loopback. During this time, it will not be possible to apply any other loops at either end of the service. If the loop needs to be removed, click on Cancel.

Hints & Tips

Hits and tips for loopback

Layer 3 Loopbacks In some cases with older test equipment, Layer 3 loopbacks can cause operational errors because of the TCP/UDP port address swapping that is applied. In the event that these are observed, the user should revert to a Layer 2 loopback test in order to confirm that the soft loops are functioning correctly on the device.
Layer 2/Layer 3 Loopbacks on Ethernet Services If a customised loopback is being applied to the end-point of a Point to Point (Ethernet) Service for which Link Loss Forwarding (LLF) with Client Signal Failure (CSF) is enabled then the port where the loop is to be applied must remain ‘enabled/alive’ in order for the ‘loopback’ to function. The CSF function will disable this port if the far-end port goes down.

In order to ensure this ‘enabled’ state is maintained then one of the following actions will be required:

The LLF CSF function must be disabled on the circuit for the duration of the loopback test

The port where the loop is to be applied must be kept alive by having the normal user terminating equipment connected