Link Aggregation

Link aggregation is a computer networking term to describe various methods of combining (aggregating) multiple network connections in parallel to increase throughput beyond what a single connection could sustain, and to provide redundancy in case one of the links fails.

Further umbrella terms used to describe the method include port trunking,link bundling, Ethernet/network/NIC bonding, or NIC teaming. These umbrella terms not only encompass vendor-independent standards such as IEEE 802.1ax Link Aggregation Control Protocol (LACP) for wired Ethernet, or the previous IEEE 802.3ad, but also various proprietary solutions.

Aggregation can be implemented at any of the lowest three layers of the OSI model. Commonplace examples of aggregation at layer 1 are power line (e.g. IEEE 1901) and wireless (e.g. IEEE 802.11) network devices that combine multiple frequency bands into a single wider one. 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, e.g. such as those of Open vSwitch. Aggregation is also possible at layer 3 in the OSI model, i.e. at the network layer (e.g. IP or IPX), using round-robin scheduling, or based on hash values computed from fields in the packet header, or a combination of these two methods. Regardless of the layer on which aggregation occurs, the network load is balanced across all links. Most methods provide failover/redundancy 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 can be done such that each interface has its own address. The former requires that both ends of a link use the same aggregation method, but has performance advantages over the latter.