How to enable ethernet 1 Gigabit advertising or troubleshoot the situation?

Question

On an Debian Stretch box, I have a Gigabit ethernet network adapter, but this mode is not active (however displayed as supported).

Does this depend on the peripheral condition (i.e. cable is not good enough, another one box has an older i.e. only 100 Mbit network controller - or can I somehow activate this mode?

Output of ethtool eth0:

Settings for eth0:
        Supported ports: [ TP MII ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Half 1000baseT/Full
        Supported pause frame use: No
        Supports auto-negotiation: Yes
        Advertised link modes:  10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
        Advertised pause frame use: Symmetric Receive-only
        Advertised auto-negotiation: Yes
        Link partner advertised link modes:  10baseT/Half 10baseT/Full
                                             100baseT/Half 100baseT/Full
        Link partner advertised pause frame use: Symmetric
        Link partner advertised auto-negotiation: Yes
        Speed: 100Mb/s
        Duplex: Full
        Port: MII
        PHYAD: 0
        Transceiver: internal
        Auto-negotiation: on
        Supports Wake-on: pumbg
        Wake-on: g
        Current message level: 0x00000033 (51)
                               drv probe ifdown ifup
        Link detected: yes

Solution

The key lines are:

    Link partner advertised link modes:  10baseT/Half 10baseT/Full
                                         100baseT/Half 100baseT/Full

This likely means that the other device (most likely a switch) is not advertising 1Gbps, or that the cable in question isn't capable of supporting 1Gbps. Check both, and verify that the cable has all eight pins connected - if in doubt, switch out the cable.

You can also use mii-tool -v to verify the information from ethtool.

Alternatively, you can use ethtool to force the NIC to 1Gbps, but be aware you may lose network connectivity:

ethtool -S eth0 speed 1000 duplex full autoneg on

Comments

[Zac67]

You can rule out the cable. A sub-standard cable will bring up the link at negotiated speed and will then cause errors. Ethernet does next to no channel testing.

[ObiwanKeTobi]

@Zac67 a sub-standard cable will cause errors, but cables with four out of eight pins (I have seen cables like this shipped with home routers and low-speed devices like monitoring cards/iLOs) would link fine but wouldn't allow 1Gbps. I found this at my own cost when blindly using cables out of my parts bin. The offending cables were swiftly chopped up and binned.

[Zac67]

Yes, you're right - just two pairs in the cable would prevent 1000BASE-T linking. I was thinking more in the line of a slightly damaged cable.