linux bridge

ubuntu bridge

vi /etc/network/interfaces

 auto br0
 iface br0 inet static
         bridge_ports eth0 eth1
         bridge_stp on
         bridge_maxwait 0
         address 192.168.2.4
         netmask 255.255.255.0
         gateway 192.168.2.2

centos bridge

vi /etc/sysconfig/network-scripts/ifcfg-br0

DEVICE="br0"
TYPE="Bridge"
ONBOOT="yes"
BOOTPROTO="static"
IPADDR="192.168.12.10"
NETMASK="255.255.255.0"
GATEWAY="192.168.12.2"
DNS1=192.168.12.2
NM_CONTROLLED="no"

vi /etc/sysconfig/network-scripts/ifcfg-em1

DEVICE=em1
BRIDGE=br0
TYPE=Ethernet
ONBOOT=yes