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Auto-negotiation and forced link mode

Auto-negotiation is a communication method and a set of steps employed by Ethernet devices connected via twisted pair cables. It enables these devices to agree on key transmission settings, including speed, duplex mode, and flow control. During this process, the connected devices initially exchange information about their capabilities concerning these settings. Afterward, they mutually select the best possible transmission mode that both devices can support effectively.

In RouterOS, auto-negotiation is always enabled by default and it selects the best possible transmission mode based on configured advertise arguments whichacepts multiple values. Other option is to force the interface into single speed, but for this property to work it requires disabled auto-negotiation.

# Enabled auto-negotiation (default) with multiple advertise values
/interface ethernet
set [ find default-name=ether2 ] advertise=100M-baseT-full,1G-baseT-full

# Disabled auto-negotiation with single speed value
/interface ethernet
set [ find default-name=ether3 ] auto-negotiation=no speed=100M-baseT-full

When it comes to SFP and QSFP type of interfaces, auto-negotiation procedure differs significantly. Prior to RouterOS version 7.12, enabled auto-negotiation attempts to guess the maximum speed of the transceiver and interface (making the advertise setting inapplicable for SFP/QSFP interfaces), or disabled auto-negotation together with speed property attempts to force the speed.

After the RouterOS version 7.12, you can now utilize the advertise bits to specify the desired link modes you want to use. The speed arguments have also been revised to provide clearer representation, as the previous values were too ambiguous. Additionally, the full-duplex setting has been removed because the new link modes already encompass the duplex options. The fec-mode setting also plays important role for establishing a working link between SFP28, QSFP+ and QSFP28 interfaces. The same mode should be used on both link ends, otherwise FEC mismatch could result in non-working link or even false link-ups. RouterOS uses a disabled FEC mode as the default setting, but it can be changed to fec74 (aka FC-FEC) or fec91 (aka RS-FEC). For more information on FEC mode options, refer to the property description.

Each interface expose all the supported link mode capabilities. They can be monitored by the "/interface ethernet monitor" command:

[admin@MikroTik] > /interface/ethernet/monitor qsfp28-1-1
                      name: qsfp28-1-1
                 supported: 10M-baseT-half,10M-baseT-full,100M-baseT-half,100M-baseT-full,1G-baseT-half,1G-baseT-full,
                            1G-baseX,2.5G-baseT,2.5G-baseX,5G-baseT,10G-baseT,10G-baseSR-LR,10G-baseCR,40G-baseSR4-LR4,
                            40G-baseCR4,25G-baseSR-LR,25G-baseCR,50G-baseSR2-LR2,50G-baseCR2,100G-baseSR4-LR4,100G-baseCR4
...

Then we can set advertise bits with the modes that we would like to use. When transceiver is inserted, RouterOS SFP handling tries to create a list of all the modes that this module and interface could support.These values can be monitored as sfp-supported and advertising:

[admin@MikroTik] > /interface/ethernet/monitor qsfp28-1-1
                      name: qsfp28-1-1
                 supported: 10M-baseT-half,10M-baseT-full,100M-baseT-half,100M-baseT-full,1G-baseT-half,1G-baseT-full,
                            1G-baseX,2.5G-baseT,2.5G-baseX,5G-baseT,10G-baseT,10G-baseSR-LR,10G-baseCR,40G-baseSR4-LR4,
                            40G-baseCR4,25G-baseSR-LR,25G-baseCR,50G-baseSR2-LR2,50G-baseCR2,100G-baseSR4-LR4,100G-baseCR4
             sfp-supported: 10G-baseSR-LR,25G-baseSR-LR,100G-baseSR4-LR4
               advertising: 10G-baseSR-LR,25G-baseSR-LR,100G-baseSR4-LR4
... 

The list of link modes are chosen like this:

link-modes = (interface-supported & advertising-bits) & sfp-supported-modes

In the end, the maximum rate of the link-modes is selected.

One application for this is to configure all of the link modes within QSFP's. The overall configuration process starts with the topmost enabled port. If the chosen mode is valid and supported, it will be applied. If that particular link mode requires multiple lanes, the advertise and speed configuration of the next interface is ignored, but the interface should remain enabled. The next available free lane or port follows a similar process.

/interface ethernet
set [ find default-name=qsfp28-1-1 ] advertise=50G-baseCR2
set [ find default-name=qsfp28-1-3 ] advertise=25G-baseCR
set [ find default-name=qsfp28-1-4 ] advertise=25G-baseCR

In the situation when a warning message appears in the log records after connecting the DAC cable or optical module, as example:

10:20:47 interface,warning sfp-sfpplus1 module auto-initialization failed, try forced-mode

This may indicate that the connected cable or module has a corrupted or bad EEPROM checksum, which causes the automatic connection configuration to fail. This functionality was introduced in RouterOS version 7.12 and may affect some links that worked in the past by mistake with wrongly assumed module/cable attributes, which could lead to various problems related to link connection and device functionality.

In such cases, you can attempt to set the link mode manually, and this might help establish a working link. The following forced port setting examples are provided:

• For DAC cables and connection speeds of 1G/10G/25G:

/interface ethernet
set [ find default-name=sfp-sfpplus1 ] auto-negotiation=no speed=1G-baseT-full
set [ find default-name=sfp-sfpplus1 ] auto-negotiation=no speed=10G-baseCR
set [ find default-name=sfp-sfpplus1 ] auto-negotiation=no speed=25G-baseCR

• For optical modules and connection speeds of 1G/10G/25G:

/interface ethernet
set [ find default-name=sfp-sfpplus1 ] auto-negotiation=no speed=1G-baseX
set [ find default-name=sfp-sfpplus1 ] auto-negotiation=no speed=10G-baseSR-LR
set [ find default-name=sfp-sfpplus1 ] auto-negotiation=no speed=25G-baseSR-LR

Properties

Sub-menu: /interface ethernet

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