Understanding Link Aggregation and LACP [FAQs]

Updated on May 15, 2021 by

What Is Link Aggregation?

Link aggregation, also known as port aggregation or link bundling, is a networking technique that involves combining multiple Ethernet links into a single logical link between two networked devices. This process allows for increased bandwidth and improved performance by leveraging the collective capacity of multiple connections. Link aggregation is commonly used in scenarios where higher throughput and fault tolerance are desired, such as network switches, servers, or network attached storage (NAS) devices.

By treating the aggregated links as a single entity, network devices and management functions can effectively utilize the combined bandwidth and distribute network traffic evenly. Link aggregation provides redundancy as well, ensuring uninterrupted connectivity in case of link failures. This technique follows IEEE standards, such as 802.3ad and 802.1AX, and is widely implemented in various networking setups, offering a cost-effective solution to enhance network performance and reliability.

What Is LAG and How Does It Work?

LAG (Link Aggregation Group) is an actual technique or instance for link aggregation. A Link Aggregation Group forms when we connect multiple ports in parallel between two switches and configure them as LAG. LAG builds up multiple links between two switches, which expands bandwidth.

Besides, it provides link-level redundancy in network failure and load-balance traffic. Even if one link fails, the remaining links between the two switches will still be running. They also take over those traffic supposed to traverse via the failed one, so data packet won’t get lost.

There are two main types of LAGs: static (manual) and dynamic LAGs.

Static LAGs require manual configuration, where an administrator creates an Eth-Trunk interface and adds member interfaces to it. This mode does not involve any negotiation protocol and relies on manual setup.

Dynamic LAGs, on the other hand, utilize the Link Aggregation Control Protocol (LACP) for automatic configuration. LACP enables network devices to negotiate the bundling of links and establish a LAG. With LACP, devices can detect link failures and switch traffic to other member links within the LAG.

It's important to note that not all devices support dynamic LAGs with LACP. Some devices only support static LAGs. It's recommended to consult the product's user manual to determine the supported LAG modes.

What Is LACP (Link Aggregation Control Protocol) and How Does It Work?

LACP is a subcomponent of IEEE 802.3ad (Link Aggregation) standard. The standard prescribes that LACP can be a method to bundle multiple physical links between network devices into a single logical link.

As a result, links that LACP enabled, can increase its logical bandwidth and network reliability without changing any network infrastructure.

Moreover, even if one link fails, under the LACP mode, the other available link members in the same LACP group will balance the load.

At the moment LACP enabled between the two switches, they will send LACPDUs (LACP data units) to each other. After receiving LACPDUs from each other, the two switches will determine which side’s system priority is higher.

Then they will have a negotiation with each other to choose the higher one to be the Actor and the lower one being the Partner. If the two switches have a similar system priority, the switch with a smaller MAC address value will be the Actor.

After the Actor selected, the two switches will choose active ports based on the port priorities of Actor’s port. However, if the Actor’s ports have the same priorities, the ports with smaller port numbers will be chosen to be active ports.

After the corresponding ports of two switches being selected, the port-channel (LACP group) is established. Then active links will load balance data to do communication.

LAG vs LACP: What’s the Difference?

LAG (link aggregation group) refers to the initial technology to realize link bundling and load balancing without any protocol involved. It is also called the manual mode, because of its working process — users need to manually create a port-channel and add member interfaces to that port-channel.

After the aggregation links being established, all those links are active links to forward data packets. If one active link fails, the other remaining active links will load balance the traffic. However, this mode can only detect disconnections of its member links, but cannot detect other faults such as link-layer faults and incorrect link connections.

LACP is a protocol for auto-configuring and maintaining LAG. Under the mode of LACP, the port-channel is created based on LACP. The LACP provides a standard negotiation mechanism for a switching device so that the switching device can automatically form and start the aggregated link according to its configuration. After the aggregated link is formed, LACP is responsible for maintaining the link status.

When the link aggregation condition is changed, LACP adjusts or removes the aggregated link. If one active link fails, the system selects a link among backup links as the active link. Therefore, the number of links participating in data forwarding remains unchanged. In addition, this mode cannot only detect disconnections of its member links, but also other faults such as link-layer faults and incorrect link connections.

LACP (Link Aggregation Control Protocol) provides advantages over static link aggregation by detecting and handling link failures, ensuring packets are not lost, and allowing mutual confirmation of the LAG configuration between devices. It quickly detects and responds to link failures, maintains the link status at the switching port even with media converters, and promptly identifies any configuration or wiring errors for improved reliability.

Should I Enable Link Aggregation?

If you have a switch with a whole lot of Gigabit Ethernet ports, you can connect all of them to another device that also has a bunch of ports and balance the traffic among these links to improve performance.

Another important reason for using link aggregation is to provide fast and transparent recovery in case one of the individual links fails. Therefore, if you care a lot about network reliability and availability, it would be better to enable link aggregation on your devices.

Link aggregation also offers the following advantages:

  • Increased reliability and availability: Link aggregation allows for dynamic and transparent reassignment of traffic to other physical links in the LAG if one link fails, ensuring continuous connectivity.

  • Efficient utilization of resources: Traffic can be load-balanced across the aggregated links, optimizing the use of physical resources and preventing congestion.

  • Enhanced bandwidth: The combined bandwidth of the aggregated links exceeds that of individual links, providing higher overall network capacity.

  • Cost-effectiveness: Link aggregation enables increased bandwidth without the need for additional hardware or costly physical network upgrades, making it a cost-effective solution.

How to Configure Link Aggregation Switch?

Link aggregation switch, or LACP switch, refers to setting up or configuring the switch to achieve the link aggregation technology. Link aggregation switch can be any switch like Gigabit Ethernet switches or 10 Gigabit switches that supports LACP. Generally, there are six steps to configure link aggregation switches:

Step 1: Add member interfaces to the channel-group.

Step 2: Set the LACP system priority and determine the Actor so that the Partner selects active interfaces based on the Actor interface priority.

Step 3: Set the upper threshold for the number of active interfaces to improve reliability. (This step is only optionally applied in CLI dynamic configuration commands.)

Step 4: Set LACP interface priorities and determine active interfaces so that interfaces with higher priorities are selected as active interfaces. (This step is only optionally applied in CLI dynamic configuration commands.)

Step 5: Create VLANs and add interfaces to the VLANs. (This step is only optionally applied in CLI dynamic configuration commands.)

Step 6: Verify the LACP configuration.

Switches that Support LAG

If you are looking for good network switches that supports LACP, here are three best-selling ones from FS, a global high-tech company providing high-speed communication network solutions and services.

  S3900-24T4S S5850-48S6Q S5860-20SQ
Management Layer Layer 2+ Layer 3 Layer 3
Port Combination 24 x 1G, 4 x 10G SFP+ 48 x 10G SFP+, 6 x 40G QSFP+ 20 x 10G SFP+, 4 x 25G SFP28, 2 x 40G QSFP+
Key Features VLAN, QoS, IGMP Snooping, Link Aggregation, IPv6,L3 Static Routing VLAN, QoS, IGMP Snooping, Link Aggregation, Static Routing, RIP, OSPF, IPv6 support, BGP/ISIS, MLAG QoS, IGMP Snooping, Link Aggregation, IPv6, L3 Static Routing, RIP, OSPF, BGP/ISIS, Stacking
Application SMB(<300 Users) Enterprise (300-2500 Users) Medium & Large Enterprise (>2500 Users)


In conclusion, link aggregation and LACP (Link Aggregation Control Protocol) offer significant benefits for network environments. Link aggregation combines multiple physical links into a logical interface, providing increased reliability, availability, and efficient utilization of resources. LACP enhances these advantages by enabling automatic configuration, fault detection, and mutual validation of LAG configurations between devices.

FS is a professional provider of communication and high-speed network system solutions, offering a wide range of high-performance network switches that support link aggregation and LACP. Find out more at FS.com.

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