Link aggregation

In computer networking, link aggregation is the combining (aggregating) of multiple network connections in parallel by any of several methods. Link aggregation increases total bandwidth beyond what a single connection could sustain, and provides redundancy where all but one of the physical links may fail without losing connectivity. A link aggregation group (LAG) is the combined collection of physical ports. Other umbrella terms used to describe the concept include trunking, bundling, bonding, channeling or teaming. Implementation may follow vendor-independent standards such as Link Aggregation Control Protocol (LACP) for Ethernet, defined in IEEE 802.1AX or the previous IEEE 802.3ad, but also proprietary protocols.

In computer networking, link aggregation is the combining (aggregating) of multiple network connections in parallel by any of several methods. Link aggregation increases total bandwidth beyond what a single connection could sustain, and provides redundancy where all but one of the physical links may fail without losing connectivity. A link aggregation group (LAG) is the combined collection of physical ports. Other umbrella terms used to describe the concept include trunking, bundling, bonding, channeling or teaming. Implementation may follow vendor-independent standards such as Link Aggregation Control Protocol (LACP) for Ethernet, defined in IEEE 802.1AX or the previous IEEE 802.3ad, but also proprietary protocols. Keep-alive mechanism for link member: (slow = 30s, fast=1s) Selectable load-balancing mode is available in some implementations LACP mode: Active: Enables LACP unconditionally. Passive: Enables LACP only when an LACP device is detected. The Linux bonding driver provides a method for aggregating multiple network interface controllers (NICs) into a single logical bonded interface of two or more so-called (NIC) slaves. The majority of modern Linux distributions come with a Linux kernel which has the Linux bonding driver integrated as a loadable kernel module and the ifenslave (if = [network] interface) user-level control program pre-installed. Donald Becker programmed the original Linux bonding driver. It came into use with the Beowulf cluster patches for the Linux kernel 2.0. Modes for the Linux bonding driver (network interface aggregation modes) are supplied as parameters to the kernel bonding module at load time. They may be given as command-line arguments to the insmod or modprobe commands, but are usually specified in a Linux distribution-specific configuration file. The behavior of the single logical bonded interface depends upon its specified bonding driver mode. The default parameter is balance-rr. Round-robin (balance-rr): Transmit alternate network packets in sequential order from the first available NIC slave through the last. This mode provides load balancing and fault tolerance. This mode can cause congestion control issues due to the packet reordering it can introduce. Active-backup (active-backup): Only one NIC slave in the bond is active. A different slave becomes active if, and only if, the active slave fails. The single logical bonded interface's MAC address is externally visible on only one NIC (port) to simplify forwarding in the network switch. This mode provides fault tolerance. XOR (balance-xor): Transmit network packets based on a hash of the packet's source and destination. The default algorithm only considers MAC addresses (layer2). Newer versions allow selection of additional policies based on IP addresses (layer2+3) and TCP/UDP port numbers (layer3+4). This selects the same NIC slave for each destination MAC address, IP address, or IP address and port combination, respectively. Single connections will have guaranteed in order packet delivery and will transmit at the speed of a single NIC. This mode provides load balancing and fault tolerance. Broadcast (broadcast): Transmit network packets on all slave network interfaces. This mode provides fault tolerance. IEEE 802.3ad Dynamic link aggregation (802.3ad, LACP): Creates aggregation groups that share the same speed and duplex settings. Utilizes all slave network interfaces in the active aggregator group according to the 802.3ad specification. This mode is similar to the XOR mode above and supports the same balancing policies. The link is set up dynamically between two LACP-supporting peers. Adaptive transmit load balancing (balance-tlb): Linux bonding driver mode that does not require any special network-switch support. The outgoing network packet traffic is distributed according to the current load (computed relative to the speed) on each network interface slave. Incoming traffic is received by one currently designated slave network interface. If this receiving slave fails, another slave takes over the MAC address of the failed receiving slave. Adaptive load balancing (balance-alb): includes balance-tlb plus receive load balancing (rlb) for IPv4 traffic and does not require any special network 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 NIC slaves in the single logical bonded interface, such that different network peers use different MAC addresses for their network packet traffic. The Linux Team driver provides an alternative to bonding driver. The main difference is that Team driver kernel part contains only essential code and the rest of the code (link validation, LACP implementation, decision making, etc.) is run in userspace as a part of teamd daemon. ===Ethernet aggregation mismatch=== Aggregation mismatch refers to not matching the aggregation type on both ends of the link. Some switches do not implement the 802.1AX standard but support static configuration of link aggregation. Therefore, link aggregation between similarly statically configured switches may work but will fail between a statically configured switch and a device that is configured for LACP. ==Examples== ===Ethernet=== On Ethernet interfaces, channel bonding requires assistance from both the Ethernet switch and the host computer's operating system, which must stripe the delivery of frames across the network interfaces in the same manner that I/O is striped across disks in a RAID 0 array. For this reason, some discussions of channel bonding also refer to Redundant Array of Inexpensive Nodes (RAIN) or to redundant array of independent network interfaces.

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