Physical port and VLAN loopback

SR Linux allows you to put a physical interface or VLAN subinterface into loopback mode to perform diagnostic testing or troubleshooting. When an interface or subinterface is in loopback mode, the datapath is changed to internally loop received traffic to the transmitting side or to loop transmitted traffic to the receiving side. The loopback feature can operate in either terminal mode or facility mode.

Terminal loopback mode (physical interfaces only)

Note: Terminal loopback mode is supported only on physical interfaces and only on 7250 IXR platforms.

In terminal mode (also called internal loopback), traffic that is normally transmitted from the system is instead redirected as if received from an external source on the same interface. If the interface receives traffic while in terminal loopback mode, the traffic is dropped.

Facility loopback mode (physical interfaces or VLAN subinterfaces)

Note: Facility loopback mode is supported on physical interfaces and VLAN subinterfaces on both the 7250 IXR and 7730 SXR platforms.

In facility mode (also called line loopback or external loopback), traffic received from an external source is redirected back to the transmitting side of the same interface or subinterface.

On a physical interface, facility loopback can be enabled on all null, dot1q, and QinQ encapsulated interfaces. On a VLAN subinterface, facility loopback can be enabled only on dot1q and QinQ encapsulated subinterfaces. VLAN loopback is not supported on system or loopback interfaces.

Facility mode also supports the option to perform a MAC swap. When MAC swap is enabled, the system swaps the source and destination Ethernet MAC addresses for unicast and unknown unicast traffic only. Broadcast and multicast traffic is dropped for both data and control traffic. Nokia recommends to enable MAC swap for MAC-VRF, IP-VRF, and default network instances.

Note: On 7250 IXR platforms, MAC swap must be configured when facility loopback is enabled.

When a physical interface is in facility loopback mode, certain port protocols including EFM-OAM, LACP, LLDP, and EAPOL (802.1x packets) are not looped back at ingress. At egress, system-generated frames for these same port protocols are transmitted to keep the protocol active and allow traffic to be sent out of the loopback-enabled interface. Any other control protocol packets are dropped.

When a VLAN subinterface is in facility loopback mode, at ingress, all frames received are looped back. However, at egress, all CPU generated traffic is dropped.

Configuring loopback mode

When an administratively enabled interface (non-breakout or breakout) is put in terminal loopback mode, the interface status immediately transitions to operationally up, even if no transceiver is installed. Similarly, if the interface is a member of a static LAG (LACP mode static) and the min-links threshold is met, the LAG immediately becomes fully functional.

If an interface fails to enter a configured loopback mode for any reason, the state value for the interface shown in the info interface loopback-mode command remains at none.

In addition, when facility loopback mode is enabled on an interface or subinterface, the existing statistics are not cleared. If you must verify the interface counters, first clear the interface statistics and then perform any required traffic tests.

To configure the loopback mode on a physical interface or VLAN subinterface, use the loopback-mode command. Supported values are: facility, none, and terminal (terminal is supported on physical interfaces only).

Set the physical port loopback mode to terminal (7250 IXR)

The following example sets the loopback mode for interface ethernet-1/1 to terminal. 

--{ candidate shared default }--[  ]--
# info interface ethernet-1/1
    interface ethernet-1/1 {
        loopback-mode terminal
    }

Set the physical port loopback mode to facility

The following example sets the loopback mode for interface ethernet-1/2 to facility and sets the MAC swap setting to true. MAC swap is supported only with facility loopback mode. 

--{ candidate shared default }--[  ]--
# info interface ethernet-1/2
    interface ethernet-1/2 {
        loopback-mode facility
        swap-src-dst-mac true
    }

Set the VLAN loopback mode to facility

The following example sets the loopback mode to facility for VLAN subinterface 1 on interface ethernet-1/3 and sets the MAC swap setting to true. 

--{ candidate shared default }--[  ]--
# info interface ethernet-1/3 subinterface 1
    interface ethernet-1/3 {
        subinterface 1 {
            vlan {
                loopback-mode facility
                swap-src-dst-mac true
            }
        }
    }