Link Aggregation Control Protocol (LACP) and Handling Elephant Flows

In the networking world, link aggregation is critical for building robust, high-performance networks. Link aggregation allows multiple physical links to be combined into a single logical link, enabling increased bandwidth, redundancy, and load balancing. However, understanding the intricacies of link aggregation control protocol (LACP) and dealing with elephant flows, which can cause imbalances, is essential for network engineers.

What is LACP?

LACP, short for Link Aggregation Control Protocol, is an optional part of the IEEE 802.3AD standard (also known as link aggregation or LAG) that defines how multiple links can be combined to form a single logical link, known as a LAG. Unlike some protocols, LACP is not a standalone protocol; instead, it works in conjunction with the link aggregation standard to modify forwarding behavior, allowing multiple active paths and efficient load balancing across the LAG.

Understanding LACP and Link Aggregation

LACP allows devices to negotiate the formation of a LAG, which is particularly useful for avoiding issues with multiple paths, such as broadcast storms and MAC flapping. It provides a dynamic mechanism for determining which links should be active and which should be in standby mode within the LAG. Additionally, LACP ensures that both ends of the link aggregation agree on the membership of the LAG.

Key Elements of LACP:

1. System ID: Each device participating in LACP has a unique identifier, typically the system MAC address, which helps prevent incorrect cabling and avoids connectivity issues.

2. Active and Standby Links: LACP dynamically selects active and standby links within the LAG based on the System IDs sent in LACP Protocol Data Units (PDUs). Active links carry traffic, while standby links are available for redundancy.

3. Load Balancing: LACP does not choose the load balancing mechanism itself; instead, it relies on the link aggregation standard (802.3AD) to define the behavior for load distribution across the active links.

Handling Elephant Flows

Elephant flows are high-volume, persistent data streams that can significantly impact traffic distribution across a link aggregation group. Since link aggregation aims to achieve even load distribution, elephant flows can disrupt this balance.

Addressing Elephant Flows:

1. Hashing Mechanism: The effectiveness of load balancing in a LAG depends on the hashing mechanism used to determine which link a packet should traverse. Vendors implement various hashing algorithms, such as hashing based on layer 2 (MAC addresses), layer 3 (IP addresses), layer 4 (TCP/UDP ports), or a combination of these. Selecting the appropriate hashing method is crucial for achieving a balanced load.

2. Variety in Traffic: The more diverse the traffic, the better the load distribution. Switches prefer variety among MAC addresses, IP addresses, and TCP/UDP port numbers. Elephant flows, which may carry repetitive traffic, can lead to a concentration of packets on specific links.

3. Utilizing Available Links: If the hashing algorithm does not effectively distribute traffic, it might underutilize one or more links while others are overwhelmed. Network engineers may consider optimizing the hashing algorithm or redistributing traffic to achieve better load balancing in such cases.


In summary, LACP plays a crucial role in link aggregation by dynamically negotiating and maintaining link status within a LAG. Although it does not select the load-balancing mechanism, choosing the right hashing algorithm is essential for efficient traffic distribution and avoiding issues like elephant flows. By understanding LACP and carefully managing traffic distribution, network engineers can build resilient, high-performance networks capable of handling varying workloads.