Implementing Network Bonding on CentOS 7

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What is Network Bonding?

Network bonding is a method of combining two or more network interfaces together into a single interface. It will increase the network throughput, bandwidth and will give redundancy. If one interface is down or unplugged, the other one will keep the network traffic up and alive. Network bonding can be used in situations wherever you need redundancy, fault tolerance or load balancing networks.

Linux allows us to bond multiple network interfaces into single interface using a special kernel module named bonding. The Linux bonding driver provides a method for combining multiple network interfaces into a single logical “bonded” interface. The behavior of the bonded interfaces depends upon the mode; generally speaking, modes provide either hot standby or load balancing services.

Types of Network Bonding

mode=0 (balance-rr)

Round-robin policy: It the default mode. It transmits packets in sequential order from the first available slave through the last. This mode provides load balancing and fault tolerance.

mode=1 (active-backup)

Active-backup policy: In this mode, only one slave in the bond is active. The other one will become active, only when the active slave fails. The bond’s MAC address is externally visible on only one port (network adapter) to avoid confusing the switch. This mode provides fault tolerance.

mode=2 (balance-xor)

XOR policy: Transmit based on [(source MAC address XOR’d with destination MAC address) modulo slave count]. This selects the same slave for each destination MAC address. This mode provides load balancing and fault tolerance.

mode=3 (broadcast)

Broadcast policy: transmits everything on all slave interfaces. This mode provides fault tolerance.

mode=4 (802.3ad)

IEEE 802.3ad Dynamic link aggregation. Creates aggregation groups that share the same speed and duplex settings. Utilizes all slaves in the active aggregator according to the 802.3ad specification.


- Ethtool support in the base drivers for retrieving the speed and duplex of each slave. – A switch that supports IEEE 802.3ad Dynamic link aggregation. Most switches will require some type of configuration to enable 802.3ad mode.

mode=5 (balance-tlb)

Adaptive transmit load balancing: channel bonding that does not require any special switch support. The outgoing traffic is distributed according to the current load (computed relative to the speed) on each slave. Incoming traffic is received by the current slave. If the receiving slave fails, another slave takes over the MAC address of the failed receiving slave.


- Ethtool support in the base drivers for retrieving the speed of each slave.

mode=6 (balance-alb)

Adaptive load balancing: includes balance-tlb plus receive load balancing (rlb) for IPV4 traffic, and does not require any special switch support. The receive load balancing is achieved by ARP negotiation. The bonding driver intercepts the ARP Replies sent by the local system on their way out and overwrites the source hardware address with the unique hardware address of one of the slaves in the bond such that different peers use different hardware addresses for the server.

Bonding on CentOS 7

  • Intel's Ethernet converged network adapter X540-T2 was placed in 2 identical machines (Storinator s45).
  • The NIC's were connected together through XS708E Un-managed Netgear Switch via 2 cat6 network cables.
  • Both machines are running CentOS 7 MATE.
  • Network Manager is not needed and can be disabled if you wish.
  • In order to bond the network ports, the first thing needed is to create a bonding config file called: **ifcfg-bond0** in the network-scripts directory.

The following lines were added to the MASTER Bonding config file:








BONDING_OPTS="mode=0 miimon=100"*

 * The IP address can be changed to whatever you wish. you can even let dhcp handle ip selection.
 * The mode can be any number from 0 to 6 - the bonding modes are described above.

Note: Different bonding modes require various configurations ie: drivers, switch support. The default mode is 0 (Round Robin) this is the mode in which you can reach speeds greater than 10Gbits/s. No switch configuring required.

Modifying the Existing 10GbE Network Interfaces:

  • Next, we need to modify the existing individual network connections in order to make them part of the bond (slaves). The image below shows the edited config file for one of the slaves. YOU MUST DO THIS FOR ALL INTERFACES YOU WISH TO BOND.

If there is no config file for the interface you are looking for, right click your network manager selectthe connection you are looking for and click "Edit". a window will pop up and all you need to do is hit "Save". Now if you go to /etc/sysconfig/network-scripts/ you will see a file called "ifcfg_Wired_connection_X" where is X the number of the connection you selected.

Example of an interface config file which is edited to be a member of the bond.

  • Once both network connections have been modified, you must restart the network.

  # systemctl restart network

  • To check to see if the bonding interface you just created is running we can type:

  # cat /proc/net/bonding/bond0

  • If everything was configured correctly, the output should look like this:

Image displaying the Bonded Interface

Testing the Bandwidth

  • Once the NIC's are successfully bonded, it is a good idea to enable jumbo frames (setting the mtu to 9000 bytes) on each interface & the bond. This is shown in the image below.
  • Iperf was used to determine the max theoretical bandwidth. - Iperf is a commonly used network testing tool that can create Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) data streams and measure the throughput of a network that is carrying them.
  • The output looks like this:
NIC bonding-wiki.png
Iperf output displaying 14Gigabits/s

14Gbit/s is the greatest bandwidth you can get out of a single Intel NIC Card. If you wish to get 20Gbit/s you must use 2 NIC Cards using only one port on each card. If you use all 4 interfaces the bandwidth will be almost 30Gbit/s!