Understanding Link Aggregation

Link aggregation is a standard technology used to combine multiple network connections in order to provide redundancy in case of single or multiple link failure. The augmented data pipe generated by aggregating N individual links behaves as a single logical high-bandwidth data path whose capacity can be up to N times the bandwidth of each individual link it comprises. Such logical connection is called a Link Aggregation Group (LAG).

Sometimes it is also called a (port) trunk, link bundle, or port channel as it bundles a number of connected physical ports together to implement a single augmented logical communication channel capable of traffic load sharing with fast re-convergence and redundancy.

Therefore, when the number of active bundled ports in a LAG changes for some reason (for example user configuration, hardware fault, etc.), traffic patterns dynamically adapt to reflect the re-balanced state of the LAG with minimal service disruption. Load splitting is achieved by hashing each packet’s L2 through L4 headers (when present) to select a physical link to follow.

Figure 7-1 - Link Aggregation Group (LAG)

Other terms commonly used to describe this technology included with servers: network interface controller (NIC) bonding or NIC teaming.

Trunks can be either statically set up or dynamically created. In the former case the LAG creation is very fast but it lacks the automation and connectivity checks of the latter case.

For link aggregation the IEEE organization has defined a standard, vendor-independent implementation with the IEEE 802.3ad specification (later transferred to the IEEE 802.1AX-2008 document and superseded by 802.1AX-2014) and has also defined a standard dynamic protocol, called Link Aggregation Control Protocol (LACP), to automate this process.