Link Aggregation, also known as bonding in Linux, uses two or more network interfaces to make one aggregate interface. This is designed for speed and redundancy.
With Link Aggregation, traffic may be spread across the multiple lines, increasing speed, or increasing fault tolerance by moving data seamlessly from one line to another in case of network failure. Combinations of these are available.
An example of setting up an LACP bonded pair in Debian Wheezy can be found in the article Xen Networking
LACP has several different modes which are combinations of the above. Some of the modes require network switches that “talk” LACP, though modes 0, 1, 5 & 6 do not.
Round-robin policy: Transmit packets in sequential order from the first available slave through the last. This mode provides load balancing and fault tolerance.
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 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.
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.
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. Note that not all transmit policies may be 802.3ad compliant, particularly inregards to the packet mis-ordering requirements of section 43.2.4 of the 802.3ad standard. Differing peer implementations will have varying tolerances for noncompliance.
Note: 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.
Adaptive load balancing: includes balance-tlb plus receive load balancing (rlb) for IPV4 traffic, and does not require any special switch support.
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 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.