Linux Tip No. 24: How to check speed/duplex of your NIC

How to determine the speed/negotiation of your NIC
a. dmesg
[root@mail ~]# dmesg | grep -i duplex
tg3: eth1: Link is up at 100 Mbps, full duplex.

b. ethtool
[root@mail ~]# ethtool eth1
Settings for eth1:
Supported ports: [ MII ]
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: 100Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 1
Transceiver: internal
Auto-negotiation: on
Supports Wake-on: g
Wake-on: d
Current message level: 0x000000ff (255)
Link detected: yes

c. mii-tool
[root@mail ~]# mii-tool eth1
eth1: negotiated 100baseTx-FD, link ok