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.
Motivation Link aggregation increases the bandwidth and resilience of Ethernet connections. Bandwidth requirements do not scale linearly. Ethernet bandwidths historically have increased tenfold each generation: 10 Mbit/s, 100 Mbit/s, 1000 Mbit/s, 10000 Mbit/s. If one started to bump into bandwidth ceilings, then the only option was to move to the next generation, which could be cost prohibitive. An alternative solution, introduced by many of the network manufacturers in the early 1990s, is to use link aggregation to combine two physical Ethernet links into one logical link. Most of these early solutions required manual configuration and identical equipment on both sides of the connection. There are three single points of failure inherent to a typical port-cable-port connection, in either a computer-to-switch or a switch-to-switch configuration: the cable itself or either of the ports the cable is plugged into can fail. Multiple logical connections can be made, but many of the higher level protocols were not designed to fail over completely seamlessly. Combining multiple physical connections into one logical connection using link aggregation provides more resilient communications.
Architecture Network architects can implement aggregation at any of the lowest three layers of the OSI model. Examples of aggregation at layer 1 (physical layer) include power line (e.g. IEEE 1901) and wireless (e.g. IEEE 802.11) network devices that combine multiple frequency bands. OSI layer 2 (data link layer, e.g. Ethernet frame in LANs or multi-link PPP in WANs, Ethernet MAC address) aggregation typically occurs across switch ports, which can be either physical ports or virtual ones managed by an operating system. Aggregation at layer 3 (network layer) in the OSI model can use round-robin scheduling, hash values computed from fields in the packet header, or a combination of these two methods. Regardless of the layer on which aggregation occurs, it is possible to balance the network load across all links. However, in order to avoid out-of-order delivery, not all implementations take advantage of this. Most methods provide failover as well. Combining can either occur such that multiple interfaces share one logical address (i.e., IP) or one physical address (i.e., MAC address), or it allows each interface to have its own address. The former requires that both ends of a link use the same aggregation method, but has performance advantages over the latter. Channel bonding is differentiated from load balancing in that load balancing divides traffic between network interfaces on per network socket (layer 4) basis, while channel bonding implies a division of traffic between physical interfaces at a lower level, either per packet (layer 3) or a data link (layer 2) basis.
IEEE link aggregation
Standardization process By the mid-1990s, most network switch manufacturers had included aggregation capability as a proprietary extension to increase bandwidth between their switches. Each manufacturer developed its own method, which led to compatibility problems. The IEEE 802.3 working group took up a study group to create an interoperable link layer standard (i.e., encompassing the physical and data-link layers both) in a November 1997 meeting. The group quickly agreed to include an automatic configuration feature, which would add in redundancy as well. This became known as Link Aggregation Control Protocol (LACP).
802.3ad As of 2000, most gigabit channel-bonding schemes used the IEEE standard of link aggregation, which was formerly clause 43 of the IEEE 802.3 standard added in March 2000 by the IEEE 802.3ad task force. Nearly every network equipment manufacturer quickly adopted this joint standard over their proprietary standards.
802.1AX The 802.3 maintenance task force report for the 9th revision project in November 2006 noted that certain 802.1 layers (such as 802.1X security) were positioned in the protocol stack below link aggregation which was defined as an 802.3 sublayer. To resolve this discrepancy, the 802.3ax (802.1AX) task force was formed, resulting in the formal transfer of the protocol to the 802.1 group with the publication of IEEE 802.1AX-2008 on 3 November 2008. As of February 2025 the current revision of the standard is 802.1AX-2020. For an overview of the history of the 802.1AX standard, see this part of the table on the IEEE 802.1 family of standards.
Link Aggregation Control Protocol Within the IEEE Ethernet standards, the Link Aggregation Control Protocol (LACP) provides a method to control the bundling of several physical links together to form a single logical link. LACP allows a network device to negotiate an automatic bundling of links by sending LACP packets to its peer, a directly connected device that also implements LACP. LACP Features and practical examples
Maximum number of bundled ports allowed in the port channel: Valid values are usually from 1 to 8. LACP packets are sent with multicast group MAC address 01:80:C2:00:00:02 During LACP detection period LACP packets are transmitted every second 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.
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