Using Multiple Ethernet Cards

Linux port bonding

Some machines, especially servers, are equipped with dual Ethernet ports on the motherboard. In order to use both ports for increased bandwidth and/or redundancy, Linux must be configured appropriately.

You should consult this very nice overview of the Linux bonding driver and the Linux Ethernet Bonding Driver HOWTO. The kernel-doc RPM also documents port bonding in the file /usr/share/doc/kernel-doc-*/Documentation/networking/bonding.txt or in

For CentOS5 Linux this is documented in 14.2.3 Channel_Bonding_Interfaces.

Loading the bonding kernel module

Read the Channel_Bonding_Interfaces manual and bonding_Module_Directives for the parameter values. Apparently it is preferred to enter bonding parameters in the file /etc/sysconfig/network-scripts/ifcfg-bond0.

For RHEL6 read Using Channel Bonding.

Our current instructions are: Add this line to /etc/modprobe.conf (not /etc/modules.conf):

alias bond0 bonding
options bond0 mode=6 miimon=100 updelay=200

The mode=6 refers to:

Sets an Active Load Balancing (ALB) policy for fault tolerance and load balancing.
Includes transmit and receive load balancing for IPV4 traffic.
Receive load balancing is achieved through ARP negotiation.

The miimon=100 refers to:

Specifies the MII link monitoring frequency in milliseconds.
This determines how often the link state of each slave is
inspected for link failures.  A value of zero disables MII
link monitoring.  A value of 100 is a good starting point.
The use_carrier option, below, affects how the link state is
determined.  See the High Availability section for additional
information.  The default value is 0.

The updelay=200 refers to:

Specifies the time, in milliseconds, to wait before enabling a
slave after a link recovery has been detected.  This option is
only valid for the miimon link monitor.  The updelay value
should be a multiple of the miimon value; if not, it will be
rounded down to the nearest multiple.  The default value is 0.

If you do not set the updelay parameter, the syslog may show this warning:

kernel: bonding: In ALB mode you might experience client disconnections upon reconnection of a link if the bonding module updelay parameter (0 msec) is incompatible with the forwarding delay time of the switch

Switch forward delay is related to the Spanning Tree Protocol (if it’s configured), see Spanning Tree Protocol Timers:

forward delay: The forward delay is the time that is spent in the listening and learning state.

Modifying network scripts on RHEL6/CentOS6

For RHEL6 read Using Channel Bonding.

Create the file /etc/modprobe.d/bonding.conf with the contents:

alias bond0 bonding

Edit the files /etc/sysconfig/network-scripts/ifcfg-* to configure bonding as described in 8.2.4. Channel Bonding Interfaces. For using DHCP the file ifcfg-bond0 should contain:

BONDING_OPTS="mode=6 miimon=100"

and the interface files ifcfg-eth0,1 should contain:

DEVICE=eth0 # or eth1

Now restart the network:

service network restart

or reboot the machine.

Setting up port bonding in SystemImager cloning

When using systemimager to clone the nodes these steps can be performed automatically using post-install scripts, e.g., /var/lib/systemimager/scripts/post-install/20q.eth_bonding_config script for the step 2.:


# Get the Systemimager variables
. /tmp/post-install/variables.txt

# Name of the central server on this network

# Correct the SystemImager eth0 config, turning eth0 into an Ethernet bonding device (bond0=eth0+eth1)
cp -p /etc/sysconfig/network-scripts/ifcfg-eth0 /tmp/ifcfg-eth0.BAK
cat <<EOF > /etc/sysconfig/network-scripts/ifcfg-eth0

# Finished

Restart network services

At this stage the network should be restarted by service network restart, or the system should be rebooted, in order to activate the bond0 device in stead of the normal eth0 device.

Port bonding troubleshooting

No DHCP response for the bond0 device

If you’ve set up the bond0 device for DHCP by BOOTPROTO=dhcp and you don’t get a DHCP response from the server, then it may be because bond0 uses the first Ethernet device (usually eth0) for DHCP. If your DHCP server is configured with the Ethernet MAC-address of another device (for example, eth1), then DHCP will fail.

This scenario happens when the Linux kernel has swapped around the Ethernet devices eth0 and eth1 opposite to what the hardware thinks. Check this by:

ifconfig -a

to see the MAC-addresses of the network interfaces.

Getting all interface MAC addresses for DHCP

The correct solution to the problem of the bond0 bonding device using a random network interface for its DHCP client is adding all MAC-addresses to your DHCP server. In this way the DHCP server will respond correctly no matter which of the bonding slave interfaces happen to be used by the DHCP client.

Of course you have to use the same DNS name (or IP address) for all of the bonding interface MAC addresses. An example from a DHCP configuration file is:

host a069-enp5s0f0 { hardware ethernet 00:24:81:f8:a9:d5; fixed-address a069.mydomain;}
host a069-enp5s0f1 { hardware ethernet 00:24:81:f8:a9:d4; fixed-address a069.mydomain;}

Please note the the DHCP host field must be unique, so the interface named has been added.

To learn the interface MAC-addresses you may use this command:

/sbin/ip -4 -o link show

We offer a script getmaclist using pdsh for gathering this information.