Jun 8, 2017

Networking bonding mode of Linux

See also:

How to bind / configure NIC teaming/bonding in CentOS / Oracle Linux

Specifies one of the bonding policies. The default is
balance-rr (round robin).  Possible values are:

balance-rr or 0

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

active-backup or 1

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.

In bonding version 2.6.2 or later, when a failover
occurs in active-backup mode, bonding will issue one
or more gratuitous ARPs on the newly active slave.
One gratuitous ARP is issued for the bonding master
interface and each VLAN interfaces configured above
it, provided that the interface has at least one IP
address configured.  Gratuitous ARPs issued for VLAN
interfaces are tagged with the appropriate VLAN id.

This mode provides fault tolerance.  The primary
option, documented below, affects the behavior of this

balance-xor or 2

XOR policy: Transmit based on the selected transmit
hash policy.  The default policy is a simple [(source
MAC address XOR'd with destination MAC address) modulo
slave count].  Alternate transmit policies may be
selected via the xmit_hash_policy option, described

This mode provides load balancing and fault tolerance.

broadcast or 3

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

802.3ad or 4

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.

Slave selection for outgoing traffic is done according
to the transmit hash policy, which may be changed from
the default simple XOR policy via the xmit_hash_policy
option, documented below.  Note that not all transmit
policies may be 802.3ad compliant, particularly in
regards to the packet mis-ordering requirements of
section 43.2.4 of the 802.3ad standard.  Differing
peer implementations will have varying tolerances for


1. Ethtool support in the base drivers for retrieving
the speed and duplex of each slave.

2. A switch that supports IEEE 802.3ad Dynamic link

Most switches will require some type of configuration
to enable 802.3ad mode.

balance-tlb or 5

Adaptive transmit load balancing: channel bonding that
does not require any special switch support.

In tlb_dynamic_lb=1 mode; the outgoing traffic is
distributed according to the current load (computed
relative to the speed) on each slave.

In tlb_dynamic_lb=0 mode; the load balancing based on
current load is disabled and the load is distributed
only using the hash distribution.

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.

balance-alb or 6

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.

Receive traffic from connections created by the server
is also balanced.  When the local system sends an ARP
Request the bonding driver copies and saves the peer's
IP information from the ARP packet.  When the ARP
Reply arrives from the peer, its hardware address is
retrieved and the bonding driver initiates an ARP
reply to this peer assigning it to one of the slaves
in the bond.  A problematic outcome of using ARP
negotiation for balancing is that each time that an
ARP request is broadcast it uses the hardware address
of the bond.  Hence, peers learn the hardware address
of the bond and the balancing of receive traffic
collapses to the current slave.  This is handled by
sending updates (ARP Replies) to all the peers with
their individually assigned hardware address such that
the traffic is redistributed.  Receive traffic is also
redistributed when a new slave is added to the bond
and when an inactive slave is re-activated.  The
receive load is distributed sequentially (round robin)
among the group of highest speed slaves in the bond.

When a link is reconnected or a new slave joins the
bond the receive traffic is redistributed among all
active slaves in the bond by initiating ARP Replies
with the selected MAC address to each of the
clients. The updelay parameter (detailed below) must
be set to a value equal or greater than the switch's
forwarding delay so that the ARP Replies sent to the
peers will not be blocked by the switch.


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

2. Base driver support for setting the hardware
address of a device while it is open.  This is
required so that there will always be one slave in the
team using the bond hardware address (the
curr_active_slave) while having a unique hardware
address for each slave in the bond.  If the
curr_active_slave fails its hardware address is
swapped with the new curr_active_slave that was