由于ifconfig命令没法看到网卡的一些状态， 以下有5种方法查看网卡状态，是否连通网线

How to check physical Network Link Status on Linux (RHEL/Centos)
原文：http://linuxsupporttips.wordpress.com/2011/08/01/how-to-check-physical-network-link-status-on-linux-rhelcentos/

There are several ways to check Link status on RHEL, 
As far i used to check by 5- ways. Refer Below

1) 
 # dmesg | grep eth
..... 
e1000: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: None 
e1000: eth1 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: None
 
2)
# mii-tool 
eth0: negotiated 100baseTx-FD, link ok 
eth1: negotiated 100baseTx-FD, link ok  

3) 
# ethtool eth0 | grep Link 
Link detected: yes 

4) 
# cat /sys/class/net/eth0/operstate 
up  

5) 
# ip link show 
2: eth0: mtu 1500 qdisc pfifo_fast qlen 1000 link/ether 00:0c:29:b0:ef:e4 brd ff:ff:ff:ff:ff:ff 
3: eth1: mtu 1500 qdisc pfifo_fast qlen 1000 link/ether 00:0c:29:b0:ef:ee brd ff:ff:ff:ff:ff:ff 

centos下检查网卡状态