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Understanding Link Aggregation and LACP [FAQs]

Understanding Link Aggregation Control Protocol

September 6, 2019

Link aggregation is an approach to combine several individual (Ethernet) links into a single logical link. Two terms often appear along with it, that is, LAG and LACP. Have you been confused by them? Let’s dig into these terms in a bit more detail now.

Feel free to jump ahead if you’ve already taken care of some parts.

1. What is LAG and How does it Work?

2. What is LACP and How does it Work?

3. LAG vs LACP: What’s the Difference?

4.Should I Enable Link Aggregation?

5. How to Configure Link Aggregation Switch?

6. Network Switches that Support LAG

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.

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 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 as 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.

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 much about network reliability and availability, it would be better to enable link aggregation on your devices.

How to Configure Link Aggregation Switch?

Link aggregation switch, or LACP switch, refer to set up or configure 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.

More details about how to configure the LACP switch or link aggregation switch, you can refer to LACP Configuration on FS S3900 Series Switches.

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)

Conclusion

Now, have you gotten a better picture of link aggregation, LAG and LACP? Do you put them into good use and find a network switch that is right to you? Link aggregation is a way of bundling several individual (Ethernet) links so that they can work as a single logical link. A group of ports combined together is called a link aggregation group, or LAG. The active monitoring protocol that allows devices to include or remove individual links from the LAG is called Link Aggregation Control Protocol (LACP).

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