ARISTA - Port Speed and Duplex

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Port Speed Configuration

no speed Command

When you manually configure a port speed using the speed command, you lock the port to that specific speed. If you later want to revert this configuration and allow the port to negotiate its speed automatically, you use the no speed command.

Example of using the no speed command:

switch# configure terminal switch(config)# interface ethernet 1 switch(config-if-Et1)# speed 10000 switch(config-if-Et1)# no speed 

In this example:

The port is initially set to 10 Gbps with the speed 10000 command.

The no speed command is then used to remove the manual speed configuration, reverting the port to auto-negotiation mode.

After executing no speed, the port will attempt to auto-negotiate the speed with the connected device based on the devices' capabilities.

Setting Port Speed Manually

To manually set the speed of an interface, use the following command:

switch# configure terminal
switch(config)# interface ethernet 1
switch(config-if-Et1)# speed 10000

In this example, the port speed is manually set to 10 Gbps (10000 Mbps). The available speed values typically include:

• 10 (10 Mbps)
• 100 (100 Mbps)
• 1000 (1 Gbps)
• 10000 (10 Gbps)
• 25000 (25 Gbps)
• 40000 (40 Gbps)

Enabling Auto-Negotiation

By default, Arista switches try to automatically negotiate the highest possible speed with the connected device. To enable auto-negotiation, use the following command:

switch(config-if-Et1)# speed auto

This allows the switch to automatically detect the maximum speed supported by the connected device and set the interface accordingly.

Duplex Configuration

Duplex settings control whether the interface operates in half-duplex or full-duplex mode.

Setting Duplex Mode

To manually configure the duplex mode of an interface, use the following command:

switch(config-if-Et1)# duplex full

In this example, the interface is set to full-duplex mode. The other option is half-duplex:

switch(config-if-Et1)# duplex half

Note that full-duplex mode is typically used for most modern networking environments, as it allows simultaneous two-way communication.

Enabling Auto-Negotiation for Duplex

Auto-negotiation for duplex mode can be enabled with the following command:

switch(config-if-Et1)# duplex auto

This setting allows the interface to automatically negotiate the best duplex mode based on the capabilities of the connected device.

Verifying Port Speed and Duplex

To verify the current speed and duplex settings for a specific interface, you can use the following show command:

switch# show interface ethernet 1

This will display information about the interface, including its speed, duplex, and other relevant statistics. The output may look something like this:

Ethernet1 is up, line protocol is up
  Hardware is Ethernet SFP, address is 00:1c:73:9e:1b:bc
  MTU 1500 bytes, BW 10000 Kbit, DLY 10 usec
  Full-duplex, 10Gb/s, link type is auto, media type is SFP+
  ...

This output shows that Ethernet1 is operating at full-duplex mode with a speed of 10 Gbps.

Troubleshooting Speed and Duplex Configuration

If there are issues with port speed or duplex, check the following:

• Ensure both devices (e.g., switches, servers, routers) are set to the same speed and duplex configuration.
• Verify that cables and transceivers are compatible with the desired speed (e.g., a 10GbE transceiver is needed for 10 Gbps).
• Use the `show interface` command to diagnose mismatched settings or errors.
• In some cases, it may be beneficial to disable auto-negotiation on both ends of the link to force a specific configuration.

Example of troubleshooting output:

switch# show interface ethernet 1
Ethernet1 is up, line protocol is up
  Hardware is Ethernet SFP, address is 00:1c:73:9e:1b:bc
  MTU 1500 bytes, BW 10000 Kbit, DLY 10 usec
  Full-duplex, 10Gb/s, link type is auto, media type is SFP+
  ...
  802.3x flow control is enabled
  10000 Mbps, full-duplex, auto-negotiation

If you see "auto-negotiation" in the output and experience issues, try setting a fixed speed and duplex configuration.

Best Practices

• Always check that both devices are configured to use the same speed and duplex settings.
• Use auto-negotiation wherever possible to ensure proper compatibility.
• If you're manually configuring ports, ensure that cables and transceivers support the desired speed.
• For higher-speed links (e.g., 10GbE or 40GbE), verify that your network equipment (e.g., servers, NICs) supports these speeds.

Useful Links

• [Arista EOS Documentation](https://www.arista.com/en/um-eos)
• [Arista Networking Commands Reference Guide](https://www.arista.com/en/um-eos-commands)
• [Ethernet Standards and Speeds](https://www.cio.com/article/287206/ethernet-standards-and-speeds-explained.html)
• [Understanding Auto-Negotiation](https://www.networkworld.com/article/2322060/understanding-auto-negotiation.html)