过程

1. 查看网卡配置

   [root@localhost ~]# ip addr
   1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
       link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
       inet 127.0.0.1/8 scope host lo
          valid_lft forever preferred_lft forever
       inet6 ::1/128 scope host
          valid_lft forever preferred_lft forever
   2: em1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
       link/ether 34:17:eb:f0:18:8f brd ff:ff:ff:ff:ff:ff
       inet 192.168.0.84/24 brd 192.168.0.255 scope global noprefixroute em1
          valid_lft forever preferred_lft forever
       inet6 240e:390:c6a:c3a0:3617:ebff:fef0:188f/64 scope global noprefixroute dynamic
          valid_lft 259182sec preferred_lft 172782sec
       inet6 fe80::3617:ebff:fef0:188f/64 scope link noprefixroute
          valid_lft forever preferred_lft forever
   3: em2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
       link/ether 34:17:eb:f0:18:90 brd ff:ff:ff:ff:ff:ff
       inet 192.169.0.10/23 brd 192.169.1.255 scope global noprefixroute em2
          valid_lft forever preferred_lft forever
       inet6 fe80::3617:ebff:fef0:1890/64 scope link noprefixroute
          valid_lft forever preferred_lft forever
   4: docker0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default
       link/ether 02:42:b1:66:84:26 brd ff:ff:ff:ff:ff:ff
       inet 172.17.0.1/16 brd 172.17.255.255 scope global docker0
          valid_lft forever preferred_lft forever
       inet6 fe80::42:b1ff:fe66:8426/64 scope link
          valid_lft forever preferred_lft forever
   12: veth4ffa8ca@if11: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master docker0 state UP group default
       link/ether ca:d7:f0:2f:42:f0 brd ff:ff:ff:ff:ff:ff link-netnsid 0
       inet6 fe80::c8d7:f0ff:fe2f:42f0/64 scope link
          valid_lft forever preferred_lft forever
   14: veth9c158d5@if13: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master docker0 state UP group default
       link/ether 76:ab:49:a9:e4:a6 brd ff:ff:ff:ff:ff:ff link-netnsid 1
       inet6 fe80::74ab:49ff:fea9:e4a6/64 scope link
          valid_lft forever preferred_lft forever
   

   这里有 em1 和 em2 两张网卡

2. 查看网卡路由

   [root@localhost ~]# ip route show
   default via 192.168.0.1 dev em1 proto static metric 100
   default via 192.169.0.1 dev em2 proto static metric 101
   172.17.0.0/16 dev docker0 proto kernel scope link src 172.17.0.1
   192.168.0.0/24 dev em1 proto kernel scope link src 192.168.0.84 metric 100
   192.169.0.0/23 dev em2 proto kernel scope link src 192.169.0.10 metric 101
   

   可以看到，em1 的 metric 大于 em2 ， metric 值越低，优先级越高。

   Metric 为路由指定所需跃点数的整数值（范围是 1 ~ 9999），它用来在路由表里的多个路由中选择与转发包中的目标地址最为匹配的路由。所选的路由具有最少的跃点数。跃点数能够反映跃点的数量、路径的速度、路径可靠性、路径吞吐量以及管理属性。Metric 的值越小，优先级越高，如果两块网卡的Metric的值相同，就会出现抢占优先级继而网卡冲突，将会有一块网卡无法连接。

3. 更改优先级

   现在要设置 em2 的优先级比 em1 的高，只需要更改 em2 网卡配置的 metric 值比 em1 的低就可以了

   修改 em2 网卡配置：

   配置文件地址：vim /etc/sysconfig/network-scripts/ifcfg-em2

   添加配置: IPV4_ROUTE_METRIC=90

   TYPE=Ethernet
   BOOTPROTO=static
   DEFROUTE=yes
   PEERDNS=yes
   PEERROUTES=yes
   IPV4_FAILURE_FATAL=no
   IPV4_ROUTE_METRIC=90
   IPV6INIT=yes
   IPV6_AUTOCONF=yes
   IPV6_DEFROUTE=yes
   IPV6_PEERDNS=yes
   IPV6_PEERROUTES=yes
   IPV6_FAILURE_FATAL=no
   NAME=em2
   UUID=23420496-e9ed-466c-a8fc-e99b091f3a00
   DEVICE=em2
   ONBOOT=yes
   IPADDR=192.169.0.10
   NETMASK=255.255.254.0
   GATEWAY=192.169.0.1
   DNS1=114.114.114.114
   

4. 重启网卡

   # 刷新配置文件
   source /etc/sysconfig/network-scripts/ifcfg-em2
   
   # 关闭 em2 网卡
   ifdown em2
   
   # 开启 em2 网卡
   ifup em2