What is bonding and how does it work
Bonding is the same as port trunking. In the following I will use the word bonding because practically we will bond interfaces as one.
Bonding allows you to aggregate multiple ports into a single group, effectively combining the bandwidth into a single connection. Bonding also allows you to create multi-gigabit pipes to transport traffic through the highest traffic areas of your network. For example, you can aggregate three megabits ports into a three-megabits trunk port. That is equivalent with having one interface with three megabytes speed.
Where should I use bonding?
You can use it wherever you need redundant links, fault tolerance or load balancing networks. It is the best way to have a high availability network segment. A very useful way to use bonding is to use it in connection with 802.1q VLAN support (your network equipment must have 802.1q protocol implemented).
What are the types of bonding available
The best documentation is on the Linux Channel Bonding Project page http://sourceforge.net/projects/bonding/
Active-backup policy: Only one slave in the bond is active. A different slave becomes active if, and only if, 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. The primary option affects the behavior of this mode.
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.
Broadcast policy: transmits everything on all slave interfaces. This mode provides fault tolerance.
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.
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.
- Prerequisite: Ethtool support in the base drivers for retrieving the speed of each slave.
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 4
In the modprobe.conf file, add the following:
alias bond0 bonding options bond0 miimon=80 mode=5
Be sure to add this before any of the network aliases
modes: mode=0 (Balance Round Robin) mode=1 (Active backup) mode=2 (Balance XOR) mode=3 (Broadcast) mode=4 (802.3ad) mode=5 (Balance TLB) mode=6 (Balance ALB)
In the /etc/sysconfig/network-scripts/ directory create the configuration file: ifcfg-bond0
DEVICE=bond0 IPADDR=<ip address> NETMASK= NETWORK= BROADCAST= GATEWAY= ONBOOT=yes BOOTPROTO=none USERCTL=no
Change the ifcfg-eth0 to participate in the new bond device:
DEVICE=eth0 ONBOOT=yes BOOTPROTO=none USERCTL=no MASTER=bond0 SLAVE=yes
Check the status of the bond.
You can use multiple bond interfaces but for that you must load the bonding module as many as there are bond links, possibly with varying options. Presuming that you want two bond interfaces, you must configure /etc/modules.conf as follow:
alias bond0 bonding options bond0 -o bond0 mode=0 miimon=100 alias bond1 bonding options bond1 -o bond1 mode=1 miimon=100
To manage the state of the bonds yourself you can use the ifenslave command. See the manpage of ifenslave for the details.
Post CentOS 4
As time has passed, edits in /etc/modprobe.conf are disfavored, in preference to writing, and the initscripts parsing, smaller configuration files that should, but are not yet required, to end in a .conf suffix. Those files are placed in: /etc/modprobe.d/ and sourced in alphabetical sequence. It makes sense to adopt the naming practice, to 'future proof' a distribution move to an 'init' successor such as systemd