Red Hat Knowledgebase

Red Hat Enterprise Linux version 3 bonding supports 7 possible "modes" for bonded interfaces. These modes determine the way in which traffic sent out of the bonded interface is actually dispersed over the real interfaces. Modes 0, 1, and 2 are by far the most common and so will be discussed below. The other modes (3-6) are not as often deployed in live servers but documentation about them can be found in the file /usr/src/linux-2.4/Documentation/networking/bonding.txt which is provided in the kernel-source rpm.

• Mode 0

  Also known as balance-rr, is a round robin transmit policy. If two real interfaces are slaves in the bond and two packets arrive destined out of the bonded interface the first will be transmitted on the first slave and the second frame will be transmitted on the second slave. The third packet will be sent on the first and so on. This mode provides load balancing between the real interfaces and provides fault tolerance because if an interface's link is lost traffic which would have used that interface will use another interface. This mode may confuse some switches because it will see the MAC address of the bond rapidly changing between the two real interfaces.

• Mode 1

  Also known as active-backup, places one of the interfaces into a backup state and will only make it active if the link is lost by the active interface. Backup is achieved by setting a NOARP flag on the backup interface. Both interfaces will have the same MAC address and will be listening for packets but only the active interface will respond to ARP requests or source any frames. The fact that both interfaces are listening can cause duplicate packets if both interfaces are connected to the same hub or physical segment. Mode 1 provides fault tolerance but offers no load balancing.

• Mode 2

  Also known as balance-xor, chooses which interface to use for sending packets based on the source and destination MAC address. The choice of interface is made by means of taking the XOR of the source and destination MAC and finding the modulo of the number of interfaces in the bond. The result is the interface to use. The bottom line is that all traffic destined for the same MAC address will use the same interface, because the source MAC address will always be that of the bond. This type of bonding is not recommended for a machine in which most of its traffic will be destined for a single default gateway, since all of the traffic for that gateway will be sent on the same real interface. This type of bonding will provide load balancing and fault tolerance, but may cause problems with some switches because of the MAC address switching quickly between physical ports.