Which tool should I trust, mii-tool and ethtool don't show me the same values

linux ethernet gigabit-ethernet ethtool

18,857

Solution 1

Since mii-tool is deprecated, I'd go for ethtool. Can't you check the other sides opinion of the link speed, btw?

mii-tool and ethtool are known to disagree in certain circumstances; you can find bugreports of that on a host of websites. What driver are you using? At least tg3 is known to be a subject of disagreement between the two tools.

UPDATE: Checked it for you locally, but it seems mii-tool can only report upto 100MiB/s. ethtool is therefore probably right in your situation ;-) Must be the reason it is deprecated, anyway...

Solution 2

Check dmesg. Most drivers will report the speed and duplex they negotiate at. For example:

dmesg | grep eth2

For example, our HP Proliants have onboard Broadcom NICs that produce kernel log messages like this:

bnx2: eth2 NIC Link is Up, 1000 Mbps full duplex, receive & transmit flow control ON

If your computer has been up for awhile and the kernel log buffer filled up with other stuff (it's a fixed-size FIFO), you can also check syslog. You don't say what distribution you're using, but many will log kernel messages to /var/log/messages or /var/log/syslog.

18,857

Dave Cheney

A chaotic neutral System Administrator with super cow powers. My weapons are: fear cynicism an almost fanatical devotion to the command line

Updated on September 17, 2022

Comments

Dave Cheney almost 2 years

Our hosting company provides all the switching infrastructure for our servers. The private network, connected to eth2 should be GbE. Recently I've been suspecting that something is misconfigured as cacti graphs never appear to show more than 100mbit of traffic on the link.

Digging further I found that mii-tool and ethtool do not agree on the link speed.

# mii-tool eth2
eth2: negotiated 100baseTx-FD flow-control, link ok
# ethtool eth2 
Settings for eth2:
    Supported ports: [ TP ]
    Supported link modes:   10baseT/Half 10baseT/Full 
                            100baseT/Half 100baseT/Full 
                            1000baseT/Half 1000baseT/Full 
    Supports auto-negotiation: Yes
    Advertised link modes:  10baseT/Half 10baseT/Full 
                            100baseT/Half 100baseT/Full 
                            1000baseT/Half 1000baseT/Full 
    Advertised auto-negotiation: Yes
    Speed: 1000Mb/s
    Duplex: Full

Which tool should I trust ?

Govindarajulu about 15 years

Since this is a bit Linux specific, I added a Linux tag

Dave Cheney about 15 years

The interface is a Broadcom. 04:00.0 Ethernet controller: Broadcom Corporation NetXtreme BCM5751 Gigabit Ethernet PCI Express (rev 21)
Bill Weiss about 15 years

mii-tool is really unhappy on 10GE interfaces. :)
David Goodwin over 8 years

ethtool and lshw both appear to be correct/reliable in my case (when mii-tool is being unreliable)