Effective network troubleshooting is a critical task for maintaining the performance and reliability of any SD-WAN (Software-Defined Wide Area Network) deployment. One of the most powerful tools for diagnosing and managing network interfaces in a Linux environment is ethtool. Whether you're dealing with link issues, poor performance, or hardware monitoring on the SD-WAN edge, ethtool provides a wealth of information that can help pinpoint the root cause of the problem.

What is ethtool?

ethtool is a command-line utility used to query and control network driver and hardware settings on Ethernet devices. It is commonly used for checking the status of interfaces, viewing statistics, and configuring parameters such as speed, duplex, and offloading settings. For SD-WAN solutions, especially those built on Linux-based platforms, this tool is invaluable for maintaining the integrity of network connections, identifying faults, and ensuring optimal performance.

Checking the Status of an Interface

To start troubleshooting, one of the first tasks is to verify the status of the network interface. With ethtool, you can quickly determine whether an interface is up and running, as well as check key details such as link status, speed, and duplex mode.

To check the status of an interface (e.g., eth0), simply use the following command:

sudo ethtool eth7

This will return detailed information about the interface, including:

• Speed: The current link speed (e.g., 1000Mb/s).

• Duplex: Whether the interface is running in full-duplex or half-duplex mode.

• Link Detected: Whether the physical connection is active (this will show yes if the cable is connected and the interface is working correctly).

Example output:

Settings for eth7:
        Supported ports: [ FIBRE ]
        Supported link modes:   10000baseT/Full
        Supported pause frame use: Symmetric
        Supports auto-negotiation: No
        Supported FEC modes: Not reported
        Advertised link modes:  10000baseT/Full
        Advertised pause frame use: Symmetric
        Advertised auto-negotiation: No
        Advertised FEC modes: Not reported
        Speed: 10000Mb/s
        Duplex: Full
        Port: Other
        PHYAD: 0
        Transceiver: internal
        Auto-negotiation: off
        Supports Wake-on: d
        Wake-on: d
        Current message level: 0x00000007 (7)
                               drv probe link
        Link detected: yes

This basic information is often enough to verify that the physical connection is intact and that the interface is functioning at the expected speed and duplex.

Querying Interface Statistics

For deeper troubleshooting, you may want to examine the interface statistics. This can help diagnose issues such as packet loss, errors, and collisions, which are common indicators of underlying problems such as hardware faults, poor cabling, or misconfigurations.

To display the statistics for a particular interface (e.g., eth0), use the command:

sudo ethtool -S eth7

This command will return various statistics such as:

• tx_packets: The number of packets transmitted.

• rx_packets: The number of packets received.

• tx_errors: The number of transmission errors.

• rx_errors: The number of receive errors.

• collisions: The number of packet collisions detected.

For example, if you notice a high number of receive errors (rx_errors), it could indicate a problem with the cable or the physical layer. High collision rates may point to duplex mismatches or a congested network.

Example output:

NIC statistics:
     rx_packets: 781732080498
     tx_packets: 779652374679
     rx_bytes: 662503952406428
     tx_bytes: 668897732576603
     rx_pkts_nic: 781732104645
     tx_pkts_nic: 779652374679
     rx_bytes_nic: 665630883238830
     tx_bytes_nic: 672166528153592
     lsc_int: 2
     tx_busy: 0
     non_eop_descs: 0
     rx_errors: 0
     tx_errors: 0
     rx_dropped: 0
     tx_dropped: 0
     multicast: 0
     broadcast: 25115
     rx_no_buffer_count: 0
     collisions: 0
     rx_over_errors: 0
     rx_crc_errors: 0
     rx_frame_errors: 0
     hw_rsc_aggregated: 0
     hw_rsc_flushed: 0
     fdir_match: 278690217146
     fdir_miss: 504312655364
     fdir_overflow: 461046
     rx_fifo_errors: 0
     rx_missed_errors: 0
     tx_aborted_errors: 0
     tx_carrier_errors: 0
     tx_fifo_errors: 0
     tx_heartbeat_errors: 0
     tx_timeout_count: 0
     tx_restart_queue: 1279
     rx_length_errors: 0
     rx_long_length_errors: 0
     rx_short_length_errors: 0
     tx_flow_control_xon: 1244
     rx_flow_control_xon: 0
     tx_flow_control_xoff: 1647
     rx_flow_control_xoff: 0
     rx_csum_offload_errors: 4106204
     alloc_rx_page: 1638228092
     alloc_rx_page_failed: 0
     alloc_rx_buff_failed: 0
     rx_no_dma_resources: 0
     os2bmc_rx_by_bmc: 0
     os2bmc_tx_by_bmc: 0
     os2bmc_tx_by_host: 0
     os2bmc_rx_by_host: 0
     tx_hwtstamp_timeouts: 0
     tx_hwtstamp_skipped: 0
     rx_hwtstamp_cleared: 0
     tx_ipsec: 0
     rx_ipsec: 0
     fcoe_bad_fccrc: 0
     rx_fcoe_dropped: 0
     rx_fcoe_packets: 0
     rx_fcoe_dwords: 0
     fcoe_noddp: 0
     fcoe_noddp_ext_buff: 0
     tx_fcoe_packets: 0
     tx_fcoe_dwords: 0
     tx_queue_0_packets: 200085552262
     tx_queue_0_bytes: 171795688562352
     tx_queue_1_packets: 192450265864
     tx_queue_1_bytes: 165424685718770
     tx_queue_2_packets: 191825615121
     tx_queue_2_bytes: 164292454780938
     tx_queue_3_packets: 195290941432
     tx_queue_3_bytes: 167384903514543
     rx_queue_0_packets: 165755278322
     rx_queue_0_bytes: 139439136327526
     rx_queue_1_packets: 210625985703
     rx_queue_1_bytes: 180751452657952
     rx_queue_2_packets: 147101594485
     rx_queue_2_bytes: 131344507075533
     rx_queue_3_packets: 258249221988
     rx_queue_3_bytes: 210968856345417

Monitoring these statistics is crucial for understanding whether the interface is operating smoothly or if there are performance-related issues that need to be addressed.

Monitoring Hardware Information | SFP+ Modules

For SD-WAN deployments that rely on high-speed links, such as fibre connections using Small Form-factor Pluggable (SFP/SFP+) modules, ethtool can also provide useful hardware information. This is particularly important when dealing with SFP+ transceivers, where hardware-level details such as temperature, voltage, and signal strength can help diagnose connectivity issues.

To retrieve hardware information about an SFP+ transceiver, use the following command:

sudo ethtool -m eth0

This command queries the Digital Diagnostics Monitoring (DDM) information for the SFP+ module installed on the specified interface. You’ll get detailed information, such as:

• Temperature: The current operating temperature of the SFP+ module.

• Voltage: The voltage level being supplied to the transceiver.

• RX Power: The received optical power level.

• TX Bias: The transmit bias current, indicating the strength of the transmitted signal.

Example output:

        Identifier                                : 0x03 (SFP)
        Extended identifier                       : 0x04 (GBIC/SFP defined by 2-                                                                             wire interface ID)
        Connector                                 : 0x07 (LC)
        Transceiver codes                         : 0x20 0x00 0x00 0x00 0x00 0x0                                                                             0 0x00 0x00 0x00
        Transceiver type                          : 10G Ethernet: 10G Base-LR
        Encoding                                  : 0x06 (64B/66B)
        BR, Nominal                               : 10300MBd
        Rate identifier                           : 0x00 (unspecified)
        Length (SMF,km)                           : 10km
        Length (SMF)                              : 10000m
        Length (50um)                             : 0m
        Length (62.5um)                           : 0m
        Length (Copper)                           : 0m
        Length (OM3)                              : 0m
        Laser wavelength                          : 1310nm
        Vendor name                               : OEM
        Vendor OUI                                : 00:0b:40
        Vendor PN                                 : ULSP311XL-CD10
        Vendor rev                                : 10
        Option values                             : 0x00 0x1a
        Option                                    : RX_LOS implemented
        Option                                    : TX_FAULT implemented
        Option                                    : TX_DISABLE implemented
        BR margin, max                            : 0%
        BR margin, min                            : 0%
        Vendor SN                                 : A311211030066
        Date code                                 : 211015
        Optical diagnostics support               : Yes
        Laser bias current                        : 33.370 mA
        Laser output power                        : 0.5116 mW / -2.91 dBm
        Receiver signal average optical power     : 0.4361 mW / -3.60 dBm
        Module temperature                        : 38.87 degrees C / 101.97 deg                                                                             rees F
        Module voltage                            : 3.2208 V
        Alarm/warning flags implemented           : Yes
        Laser bias current high alarm             : Off
        Laser bias current low alarm              : Off
        Laser bias current high warning           : Off
        Laser bias current low warning            : Off
        Laser output power high alarm             : Off
        Laser output power low alarm              : Off
        Laser output power high warning           : Off
        Laser output power low warning            : Off
        Module temperature high alarm             : Off
        Module temperature low alarm              : Off
        Module temperature high warning           : Off
        Module temperature low warning            : Off
        Module voltage high alarm                 : Off
        Module voltage low alarm                  : Off
        Module voltage high warning               : Off
        Module voltage low warning                : Off
        Laser rx power high alarm                 : Off
        Laser rx power low alarm                  : Off
        Laser rx power high warning               : Off
        Laser rx power low warning                : Off
        Laser bias current high alarm threshold   : 110.000 mA
        Laser bias current low alarm threshold    : 1.000 mA
        Laser bias current high warning threshold : 100.000 mA
        Laser bias current low warning threshold  : 2.000 mA
        Laser output power high alarm threshold   : 1.5849 mW / 2.00 dBm
        Laser output power low alarm threshold    : 0.1585 mW / -8.00 dBm
        Laser output power high warning threshold : 1.2589 mW / 1.00 dBm
        Laser output power low warning threshold  : 0.1995 mW / -7.00 dBm
        Module temperature high alarm threshold   : 100.00 degrees C / 212.00 de                                                                             grees F
        Module temperature low alarm threshold    : -40.00 degrees C / -40.00 de                                                                             grees F
        Module temperature high warning threshold : 95.00 degrees C / 203.00 deg                                                                             rees F
        Module temperature low warning threshold  : -35.00 degrees C / -31.00 de                                                                             grees F
        Module voltage high alarm threshold       : 3.6000 V
        Module voltage low alarm threshold        : 2.8000 V
        Module voltage high warning threshold     : 3.5000 V
        Module voltage low warning threshold      : 2.9000 V
        Laser rx power high alarm threshold       : 1.9953 mW / 3.00 dBm
        Laser rx power low alarm threshold        : 0.0115 mW / -19.39 dBm
        Laser rx power high warning threshold     : 1.0000 mW / 0.00 dBm
        Laser rx power low warning threshold      : 0.0229 mW / -16.40 dBm

This information is particularly useful when troubleshooting fibre links where issues like poor signal strength or overheating could lead to intermittent connectivity problems or outright failures.

Practical Use on SD-WAN Edge Devices

In SD-WAN deployments, particularly those running on Linux-based platforms, ethtool can be used to ensure that edge devices are functioning correctly. For example, if you’re experiencing performance issues on a specific link, you can use ethtool to verify whether the issue is due to link degradation, duplex mismatches, or hardware problems like a faulty SFP+ module.

Additionally, because SD-WAN solutions often rely on multiple last-mile connections (such as fibre, fixed wireless, or mobile networks), ensuring that the interfaces for these connections are operating at their optimal settings is crucial. With ethtool, you can easily verify the status of each interface, check for errors, and ensure that each connection is delivering the performance your business needs.

Wrap

ethtool is an indispensable tool for troubleshooting and managing network interfaces on SD-WAN edge devices. Its ability to check the status of an interface, query detailed statistics, and monitor hardware-level information like SFP+ transceivers makes it a powerful utility for network engineers.

For SD-WAN deployments, where multiple connections and high-speed links are essential for business continuity, ethtool can help maintain the health of your network by providing visibility into the critical metrics that matter most. Whether you’re ensuring optimal link performance or diagnosing hardware faults, ethtool ensures your network runs smoothly and efficiently.

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Ronald Bartels ensures that Internet inhabiting things are connected reliably online at Fusion Broadband South Africa - the leading specialized SD-WAN provider in South Africa. Learn more about the best SD-WAN in the world: 👉Contact Fusion🚀

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