最近遇到一个问题，一台服务器的千兆网口只有百兆速度，记录下定位排查的过程
环境：ubuntu 18.04
使用atop命令查看网口速度
没有atop使用apt-get install atop安装下
执行atop
发现2个网口，一个1000m，一个100m
强制修改网口匹配速度
执行**“sudo ethtool eno1** ”，查看网口匹配情况
Settings for eno1:
Supported ports: [ TP ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Supported pause frame use: Symmetric
Supports auto-negotiation: Yes
Supported FEC modes: Not reported
Advertised link modes: 100baseT/Full
Advertised pause frame use: Symmetric
Advertised auto-negotiation: Yes
Advertised FEC modes: Not reported
Speed: 100Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 1
Transceiver: internal
Auto-negotiation: on
MDI-X: on (auto)
Supports Wake-on: pumbg
Wake-on: g
Current message level: 0x00000007 (7)
drv probe link
Link detected: yes
将eno1强制设置为1000m自协商全双工，并重启网口
sudo ethtool -s eno1 speed 1000 duplex full autoneg on;
sudo ethtool -r eno1;
这时神奇的情况出现了：断网了！！！
还好有一个备用网口插着网线，这时候派上用场了
通过另一个网口登陆查看eno1网口的匹配情况
sudo ethtool eno1 ─╯
Settings for eno1:
Supported ports: [ TP ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Supported pause frame use: Symmetric
Supports auto-negotiation: Yes
Supported FEC modes: Not reported
Advertised link modes: 1000baseT/Full
Advertised pause frame use: Symmetric
Advertised auto-negotiation: Yes
Advertised FEC modes: Not reported
Speed: Unknown!
Duplex: Unknown! (255)
Port: Twisted Pair
PHYAD: 1
Transceiver: internal
Auto-negotiation: on
MDI-X: on (auto)
Supports Wake-on: pumbg
Wake-on: g
Current message level: 0x00000007 (7)
drv probe link
Link detected: no
发现Speed和Duplex均为Unknown
然后把网口设置回100m，网口又通了
sudo ethtool -s eno1 speed 100 duplex full autoneg on;sudo ethtool -r eno1;
联系网管，交换机侧设置成1000m，网又断了，设置成100m，就通了
总结下：
无论服务器还是交换机，只要设置成1000m，就断网，设置成100m，又恢复了
网上查资料，很可能是网线的原因，只能等下次去机房排查了。
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