Overview
Loopback is an effective trouble shooting and isolation method that is commonly used by traditional technologies such as SONET/SDH, T1/E1 and so many others. In order for Ethernet to replace the old technologies, it must support loopback. Ethernet/Link OAM defines
procedures to put a remote entity into loopback mode using a loopback-control OAMPDU.
When an OAM entity is in loopback mode, every frame received is transmitted back on that same port except for OAMPDUs and pause frames. When an OAM entity is configured in Passive mode, it can only respond to, but not initiate, a loopback request. In order to both initiate and respond to a loopback request, it must be configured in Active mode.
Figure 84-Ethernet/Link OAM Loopback PDU Format
Objective
The test objective is to discover whether or not the DUT can respond to and/or initiate a loopback request. Data plane traffic is used to verify the hardware status.
Setup
One or more Ixia test ports can be used to verify DUT’s OAM loopback capability.
Figure 85-Link OAM Remote Loopback Test Setup
Step-by-Step Instructions
Repeat steps 1 thru 5 in the Ethernet/Link OAM Discovery test (page 55) if needed for a quick emulation setup using the Link-OAM protocol wizard.
Configure Ixia as Active mode.
Figure 86-Configure Ixia Link-OAM in Active Operation Mode
Verify the DUT is also in Active mode and, more importantly, the DUT indeed supports Remote Loopback.
Figure 87-Verify DUT in Active Mode and Support of Remote Loopback
To send a loopback request, first select the Learned Info. Then choose Enable Loopback and click Send Loopback. To confirm DUT loopback status, click Refresh and verify Remote Mux Action and Remote Parser Action. Also confirm loopback status from the DUT — both before and after the request.
Figure 88-Sequence to Send a Loopback Request
Figure 89-DUT Status Before Loopback Request
Figure 90-DUT Status After Loopback Request
Ultimately, you will need to verify the loopback by use of data plane traffic. The simplest way to do this is to click Quick Flow Groups and add 1 flow group to the port in use. A quick flow group will be generated.
Figure 91-Use Quick Flow Group to Create a Flow Group
You can quickly modify the content of the frame by clicking the Encapsulation Editor and modifying the Src/Dest MAC address as appropriate. Add any high layer protocols as you wish.
Figure 92-Modify Frame Contents As Needed
Click L2-L3 Traffic to push flow group definition to Ixia hardware and then click the green arrow to start sending traffic. Verify the traffic from Statistics > Port Statistics. As expected, Frames TX and Frames RX, as well as Frames Tx Rate and Frames Rx Rate are matching. A small variant is due to the OAMPDUs.
Figure 93-Push Flow Group Definition to Ixia HW and Start Traffic
Figure 94-Traffic Statistics Clearly Shows Loopback by DUT
To disable the loopback, simply go to Learned Info and click to select Disable Loopback command, then click Send Loopback. The DUT status can be verified in many ways — the simplest and most reliable one is to check traffic stats. The port used for sending a loopback command no longer receives traffic; and the other port is receiving traffic now since the DUT is a layer 2 switch, in this case, and it is broadcasting frames to all ports.
Disable Loopback
Figure 95-Traffic Statistics Shows Disable of Loopback
In some cases a Loopback command cannot be sent from Ixia due to local unstable state as indicated by the error message Packet Sent: 0 Error: Discovery not in stable state. To fix this, you can go to the Advanced tab and select Override Local Stable and then Send Updated Parameters.
Figure 96-Trouble Shoot if Loopback Can’t be Sent
Figure 97-Override Local Stable Condition
To test the DUT’s ability to issue a loopback request, you can type the appropriate CLI from the DUT and then go to the Ixia port and check on the Learned Info > Local Information. It should show the port is in LB state with DISCARD mux action. Should you decide to send traffic to test he loopback, make sure you set the Ethernet Type to match the traffic you’re sending from the DUT.
Figure 98-Verify DUT’s Ability to Send Loopback Request
Figure 99-Configure the Ethernet Type to Match DUT’s Traffic Type
Test Variables
Any of the following variables may be used to verify DUT Link OAM Loopback functions:
Number of test ports — if there is a need to verify OAM Loopback across multiple DUT ports
Operation Mode — Active or Passive — Only Active mode supports Loopback request.
Passive mode can only respond but can’t initiate.
Loopback request in conjunction with Remote Failure Indication flags
Loopback in conjunction with Capability flags
Loopback only on data but not on OAMPDUs and Pause frames
Traffic rate and packet size
Results Analysis
Loopback state can be verified in multiple ways. The DUT usually has a CLI to clear the display interface state. Ixia learned info will show all discovered info including loopback state. Most importantly, loopback can be confirmed using data plane traffic.
Troubleshooting and diagnostics
Problem DescriptionCannot display learned info from DUT
Check to make sure either the DUT or Ixia emulator is configured in Active mode. If both are configured in passive mode, none of them will start the discovery process, so no learned info will be displayed.
DUT won’t respond to Ixia loopback reqeust
When Ixia is in Local Unstable state during discovery phase with DUT, it will not be able to send Loopback command. To fix it, you can override the local stable state via the Advanced function.
Ixia cannot loopback traffic to DUT
Most likely the DUT is sending/forwarding traffic with a different Ethernet Type value than default “0xFFFF”.
Conclusions
Ethernet/Link OAM Loopback allows the remote end to loopback the traffic for efficient trouble shooting in a real network. Ixia’s Link OAM emulation provides the ability to verify both the DUT’s response to a loopback request and its ability to initiate a loopback request. The loopback state can be verified by multiple means including use of data plane traffic.
DUT Config Excerpt
switchport mode access
ethernet oam remote-loopback supported ethernet oam