centos7将网卡加入到网桥后， Missing config file br-ex，网卡无法正常启动问题解决

原来的/etc/sysconfig/network-scripts/ifcfg-ens2 

 HWADDR=68:05:CA:11:XX:XX
 TYPE=Ethernet
 BOOTPROTO=none
 NAME=ens2
 UUID=bc723ed2-3196-491b-8e25-bf4a56a63c81
 ONBOOT=yes

/var/log/message

 Nov 25 13:42:50 localhost network: Bringing up interface ens2:  Error: either "dev" is duplicate, or "br-ex" is a garbage.
 Nov 25 13:42:50 localhost network: cat: /sys/class/net/ens2: Is a directory
 Nov 25 13:42:50 localhost network: cat: br-ex/ifindex: No such file or directory
 Nov 25 13:42:50 localhost network: /etc/sysconfig/network-scripts/ifup-eth: line 273: 1000 + : syntax error: operand expected (error
  token is "+ ")
 Nov 25 13:42:50 localhost network: ERROR    : [/etc/sysconfig/network-scripts/ifup-aliases] Missing config file br-ex.
 Nov 25 13:42:50 localhost /etc/sysconfig/network-scripts/ifup-aliases: Missing config file br-ex.

网卡启动的脚本分别是

/etc/init.d/netwrok  -> /usr/sbin/ifup -> /etc/sysconfig/network-scripts/network-functions 

                                                               -> /etc/sysconfig/network-scripts/ifup-eth

通过跟踪各个变量发现问题是系统将DEVICE变量设置为了“ens2 br-ex2”
所以才会出现上述奇怪的log，而正确的DEVICE应该就为“ens2”

主要原因是在ifcfg-ens2 文件中没有DEVICE参数，network-functions 会根据mac地址找设备，但br-ex的Mac地址和ens2相同，所以得到的DEVICE就是错误。

解决办法就是增加DEVICE参数，修改后的/etc/sysconfig/network-scripts/ifcfg-ens2 如下

 DEVICE=ens2