Configuring Static Pre-Emphasis (Signal Integrity) Port Settings

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The SerDes (serializer/deserializer) component in switch ports has several associated Signal Integrity (SI) parameters that help in shaping the output into a clean and well-defined signal based on the transmission line characteristics of the signal traces. This output shaping is used to cancel-out factors such as reflection and signal attenuation and enables the receiver to get a clean and well-defined signal. Collectively, these signal integrity parameters are also known as pre-emphasis settings.

If the pre-emphasis settings are incorrect, you may encounter unacceptable bit error rate (BER), especially when the port speed scales to higher values. The pre-emphasis settings vary per port  based on the circuit board layout, the operational speed of the port, and the transmission medium (cable type and length).

Note: Refer to the cable manufacturer documentation where the required pre-emphasis values are defined.

This feature is not available on the following platforms:

In earlier releases, NetVisor OS had limitations to dynamically change the  pre-emphasis settings based on the  configured port number and port speed.  However, from NetVisor OS version 6.0.0 onward, you can  override the default settings for a specified set of ports by using the port-cable-type-modify port <port-list> type [optical|dac] command. For the cable-type parameter, select either dac cable setting (for copper DAC cable, which is the default setting) or optical cable setting (for optical transceivers).

The port cable type database saves the details of the cable type you had configured by using the above CLI. However, if the cable type is not specified, then a default DAC cable length value of 3m is assumed for the port. These settings are then saved to the hardware database (registers).

When you modify the port speed or the cable type, the signal integrity or pre-emphasis settings are written to the hardware. If the port is already enabled, then the port gets disabled and then re-enabled while the settings are being written to the hardware. You are prompted to proceed with the changes prior to the port being disabled and re-enabled. Additionally note that, when a port that you want to modify is part of a port group, then all the ports in that port group are modified with the new signal integrity settings.

The DAC settings in NetVisor OS is mapped to the 3m DAC settings, which is the current default on all Arista supported switches, except Z9100-ON.

Note - Guidelines for Z9100-ON platforms:

• During a fresh installation of NetVisor OS 6.0.0, by default, all the ports are automatically set to optical cable type. 
• While modifying the cable type, out of all the SFP ports, you cannot modify ports 129 and 130 (bezel ports 33 and 34)  to dac cable type due to a hardware limitation on the Z9100-ON platform.

To change the cable type, and consequently, the pre-emphasis (Signal Integrity) settings for a specified port on any platform, use the command:

CLI (network-admin@switch) > port-cable-type-modify port <port-list> type optical|dac

To view the currently applied cable type settings for all ports, use the command:

CLI (network-admin@switch) > port-cable-type-show

Example: Pre-Emphasis Settings Configuration

Let us consider an example to further explain the changes implemented by this feature.  

First verify the current port and cable type settings on a switch by using the port-cable-type-show command. From NetVisor OS version 6.0.0, only optical and dac cable types are supported.  

CLI (network-admin@switch*) > port-cable-type-show

port type        cable-sub-type 

---- ----------- -------------- 

none dac         ca-s           

none dac         ca-l           

none dac         ca-n           

none dac         default        

none optical     default        

1-24 sys-default default     

The default string in the cable-sub-type column is used to display which settings have not been changed by the user.

Now, modify the settings for port 1  by using the below command below (See the warning message regarding the application of the settings to all member ports in the port group) and confirm to proceed:

CLI (network-admin@switch*) > port-cable-type-modify port 1-4 type optical

Warning: updating SI settings will apply to all ports in a port group and will flap enabled ports

Please confirm y/n (Default: n):y

Updated SI settings for port(s) 1-4 to optical

The system log file and  nvOSd log file are automatically updated to reflect the Signal Integrity (SI) setting changes.

View the updated settings by using the command:

CLI (network-admin@switch*) > port-cable-type-show

port type        cable-sub-type 

---- ----------- -------------- 

none dac         ca-s           

none dac         ca-l           

none dac         ca-n           

none dac         default        

1-4  optical     default        

5-24 sys-default default 

To reset all the ports to dac cable type, use this command with the port all parameter:

CLI (network-admin@switch*) > port-cable-type-modify port all type dac

Warning: updating SI settings will apply to all ports in a port group and will flap enabled ports

Please confirm y/n (Default: n):y

Updated SI settings for port(s) 1-24 to dac

View the changed settings again by using the command:

 

CLI (network-admin@switch*) > port-cable-type-show

port type        cable-sub-type 

---- ----------- -------------- 

none dac         ca-s           

none dac         ca-l           

none dac         ca-n           

1-24 dac         default        

none optical     default        

none sys-default default 

Note: For platforms with copper RJ-45 ports such as S4148T-ON, an invalid  message is displayed while modifying the port cable type to optical cable type for the copper ports. For example,

CLI (network-admin@S4148T) > port-cable-type-modify port all type optical

Warning: updating SI settings will apply to all ports in a port group and will flap enabled ports

Please confirm y/n (Default: n):y

SI settings invalid for port(s) 1-48 for optical

Updated SI settings for port(s) 49-56,65-72 to optical

View the details using the show command:

 

CLI (network-admin@S4148T) > port-cable-type-show port all

switch port   type        cable-sub-type

------ ------ -------     --------------

S4148T none   dac         ca-s

S4148T none   dac         ca-l

S4148T none   dac         ca-n

S4148T 1-48   dac         default

S4148T 49-72  optical     default

S4148T none   sys-default default 

Note: This feature’s settings are applied when you modify either the port speed or the cable type for a  specified port. That is, the pre-emphasis (SI) settings in NetVisor OS are applied during the initial port configuration when the port speed is modified by using the port-config-modify command, and also when the cable type is modified using the port-cable-type-modify command.

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Caution: This feature enables you to modify the pre-emphasis settings by modifying the cable-type and port speed. However, the link quality could be impacted if incorrect settings are chosen and caution must be taken to ensure correct settings are configured.

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Guidelines and Limitations

The following guidelines and limitations should be considered while configuring the pre-emphasis settings on a switch port:

• The pre-emphasis settings are written to hardware whenever you change the cable type using the port-cable-type-modify command.
• The pre-emphasis settings are written to hardware whenever you change the port speed using the port-config-modify command.
• For Z9100-ON platforms with copper SFP ports, the cable type, dac is not accepted by NetVisor OS. If you try to change the cable type for a single SFP copper port by using the port-cable-type-modify command, then an error is displayed as follows:

CLI (network-admin@switch) > port-cable-type-modify port 129 type dac

Warning: updating SI settings will apply to all ports in a port group and will flap enabled ports

Please confirm y/n (Default: n):y

port-cable-type-modify: Invalid cable type setting

• However, if both port all and type optical parameters are used on a switch with copper ports, then the settings are selectively applied only to the optical ports, and no error is displayed.
• For those platforms that have hidden ports, such as S4148T-ON with 48 x 10GBaseT and 4 x 100 GbE ports, the cable type changes are skipped when port all parameter is specified in the port-cable-type-modify command.
• For platforms on which no pre-emphasis settings are configured on the ports, a minimum port speed of 10 Gbps is assumed.
• While this feature allows you to select the pre-emphasis settings, the link quality may get impacted if incorrect settings are applied. Ensure to apply the correct pre-emphasis settings  during initial configuration.
• Currently the 3m DAC setting is implemented for all DAC cable lengths.

Related Commands

CLI (network-admin@nru03-switch) > port-config-modify