9.3. Using the Command Line Interface (CLI)

9.3.1. Check if Bridging Kernel Module is Installed

In Red Hat Enterprise Linux 7, the bridging module is loaded by default. If necessary, you can make sure that the module is loaded by issuing the following command as root: 
~]# modprobe --first-time bridge
modprobe: ERROR: could not insert 'bridge': Module already in kernel
To display information about the module, issue the following command: 
~]$ modinfo bridge
See the modprobe(8) man page for more command options.

9.3.2. Create a Network Bridge

To create a network bridge, create a file in the /etc/sysconfig/network-scripts/ directory called ifcfg-brN, replacing N with the number for the interface, such as 0.
The contents of the file is similar to whatever type of interface is getting bridged to, such as an Ethernet interface. The differences in this example are as follows:
  • The DEVICE directive is given an interface name as its argument in the format brN, where N is replaced with the number of the interface.
  • The TYPE directive is given an argument Bridge. This directive determines the device type and the argument is case sensitive.
  • The bridge interface configuration file is given an IP address whereas the physical interface configuration file must only have a MAC address (see below).
  • An extra directive, DELAY=0, is added to prevent the bridge from waiting while it monitors traffic, learns where hosts are located, and builds a table of MAC addresses on which to base its filtering decisions. The default delay of 15 seconds is not needed if no routing loops are possible.

Example 9.1. Example ifcfg-br0 Interface Configuration File

The following is an example of a bridge interface configuration file using a static IP address: 
DEVICE=br0
TYPE=Bridge
IPADDR=192.168.1.1
PREFIX=24
BOOTPROTO=none
ONBOOT=yes
DELAY=0
To complete the bridge another interface is created, or an existing interface is modified, and pointed to the bridge interface.

Example 9.2. Example ifcfg-ethX Interface Configuration File

The following is an example of an Ethernet interface configuration file pointing to a bridge interface. Configure your physical interface in /etc/sysconfig/network-scripts/ifcfg-ethX, where X is a unique number corresponding to a specific interface, as follows: 
DEVICE=ethX
TYPE=Ethernet
HWADDR=AA:BB:CC:DD:EE:FF
BOOTPROTO=none
ONBOOT=yes
BRIDGE=br0
Optionally specify a name using the NAME directive. If no name is specified, the NetworkManager plug-in, ifcfg-rh, will create a name for the connection profile in the form “Type Interface”. In this example, this means the bridge will be named Bridge br0. Alternately, if NAME=bridge-br0 is added to the ifcfg-br0 file the connection profile will be named bridge-br0.

Note

For the DEVICE directive, almost any interface name could be used as it does not determine the device type. TYPE=Ethernet is not strictly required. If the TYPE directive is not set, the device is treated as an Ethernet device (unless its name explicitly matches a different interface configuration file).
The directives are case sensitive.
Specifying the hardware or MAC address using the HWADDR directive will influence the device naming procedure as explained in Chapter 11, Consistent Network Device Naming.

Warning

If you are configuring bridging on a remote host, and you are connected to that host over the physical NIC you are configuring, consider the implications of losing connectivity before proceeding. You will lose connectivity when restarting the service and may not be able to regain connectivity if any errors have been made. Console, or out-of-band access is advised.
To open the new or recently configured interfaces, issue a command as root in the following format: 
ifup device
This command will detect if NetworkManager is running and call nmcli con load UUID and then call nmcli con up UUID.
Alternatively, to reload all interfaces, issue the following command as root: 
~]# systemctl restart network
This command will stop the network service, start the network service, and then call ifup for all ifcfg files with ONBOOT=yes.

Note

The default behavior is for NetworkManager not to be aware of changes to ifcfg files and to continue using the old configuration data until the interface is next brought up. This is set by the monitor-connection-files option in the NetworkManager.conf file. See the NetworkManager.conf(5) manual page for more information.

9.3.3. Network Bridge with Bond

An example of a network bridge formed from two or more bonded Ethernet interfaces will now be given as this is another common application in a virtualization environment. If you are not very familiar with the configuration files for bonded interfaces, see Section 7.4.2, “Create a Channel Bonding Interface”
Create or edit two or more Ethernet interface configuration files, which are to be bonded, as follows: 
DEVICE=ethX
TYPE=Ethernet
SLAVE=yes
MASTER=bond0
BOOTPROTO=none
HWADDR=AA:BB:CC:DD:EE:FF

Note

Using ethX as the interface name is common practice but almost any name could be used.
Create or edit one interface configuration file, /etc/sysconfig/network-scripts/ifcfg-bond0, as follows: 
DEVICE=bond0
ONBOOT=yes
BONDING_OPTS='mode=1 miimon=100'
BRIDGE=brbond0
For further instructions and advice on configuring the bonding module and to view the list of bonding parameters, see Section 7.6, “Using Channel Bonding”.
Create or edit one interface configuration file, /etc/sysconfig/network-scripts/ifcfg-brbond0, as follows: 
DEVICE=brbond0
ONBOOT=yes
TYPE=Bridge
IPADDR=192.168.1.1
PREFIX=24
We now have two or more interface configuration files with the MASTER=bond0 directive. These point to the configuration file named /etc/sysconfig/network-scripts/ifcfg-bond0, which contains the DEVICE=bond0 directive. This ifcfg-bond0 in turn points to the /etc/sysconfig/network-scripts/ifcfg-brbond0 configuration file, which contains the IP address, and acts as an interface to the virtual networks inside the host.
To open the new or recently configured interfaces, issue a command as root in the following format: 
ifup device
This command will detect if NetworkManager is running and call nmcli con load UUID and then call nmcli con up UUID.
Alternatively, to reload all interfaces, issue the following command as root: 
~]# systemctl restart network
This command will stop the network service, start the network service, and then call ifup for all ifcfg files with ONBOOT=yes.

Note

The default behavior is for NetworkManager not to be aware of changes to ifcfg files and to continue using the old configuration data until the interface is next brought up. This is set by the monitor-connection-files option in the NetworkManager.conf file. See the NetworkManager.conf(5) manual page for more information.