LACP Link Aggregation: Is Your Network Missing Out?

Introduction to LACP link aggregation

LACP link aggregation is a standard networking protocol that bundles multiple physical links into a single logical link. This process of bundling multiple physical links into one logical link is known as link aggregation. IEEE standardized it under 802.3ad in 2000, But later, in 2008, IEEE incorporated 802.3ad into the broader IEEE 802.1AX standard. LACP can be configured on most vendor devices due to its open standardization, but implementation details may vary.

What is LACP in Networking?

It is a protocol developed by IEEE  that automates the configuration and management of link aggregation. LACP was developed after Cisco’s proprietary PAgP protocol. It combines several physical connections and makes them into a single logical link. LACP link aggregation protocol is the most widely used one due to its easy configuration and vendor neutrality. It plays an important role in redundancy, load balancing, increased bandwidth, and high availability.

How LACP Works

LACP works by exchanging control packets between connected devices such as switches, routers, servers, for negotiating LACP link aggregation. It has the following three phases for its working;

• 1 Discovery & negotiation: LACP configured devices exchange LACP Data Units or simply LACPDUs and confirm compatible links.
• 2 Link Selection & Bundling: After the exchange of LACPDUs, LACP dynamically selects and groups active links for bundling.
• 3 Monitoring and Failover: If any links fail in the etherchannel, LACP automatically redistributes traffic across available links to prevent downtime.

Role of LACP in networking

The role of link aggregation control protocol is as follows;

Preventing single point of failure: How LACP ensures redundancy

Increase bandwidth: How it combines multiple links for higher throughput.

Manage links dynamically: How LACP adapts to link changes.

Improved network efficiency: How it optimizes traffic distribution.

LACP Switch Operations

A LACP switch constantly monitors the etherchannel links, which ensures that all physical links work correctly. It automatically detects any failure in the aggregated links, and it reassigns traffic to the other paths in the etherchannel for ensuring uninterrupted connectivity.

Active vs Passive LACP: Key Differences

LACP has two operational modes, which are:

• Active Mode: This mode constantly sends LACP packets to negotiate link aggregation.
• Passive Mode: While this mode responds to the LACP packets, but it doesn’t initiate negotiation.

Real-World Use Cases

LACP link aggregation protocol is widely used in the following scenarios;

• Data Centers: Aggregation links have more importance in the data centers network for efficient load balancing.
• Enterprise Networks: Etherchannel ensures redundancy between switches and core network devices.
• Service Providers: There is a large amount of data flowing through ISP networks, which LACP network is crucial for streamlining operations.

Cloud Environments: LACP network optimizes performance in multi-tenant cloud setups.

LACP vs PAgP: Which One Should You Use?

We can analyze the differences through the following table;

Feature	LACP	PAgP
Vendor Support	IEEE 802.3ad	Cisco Proprietary
Modes	Active, Passive	Auto, Desirable
Flexebility	Works across different vendors	Limited to Cisco devices
Fault Tolerance	High	Moderate

LACP protocol is suitable if there are multiple switches exists of different vendor, but if all the switches are from Cisco, then use PAgP.

When to Choose LACP over PAgP

LACP is the industry standard protocol due to which every vendor can easily adopt it. However, PAgP is the Cisco proprietary protocol which only works with Cisco switches.

Configure LACP Protocol

Take two switches for LACP protocol configuration as below in the screenshot.

First of all configure basic settings such as vtp domain, vtp server, and trunking between both the switches on Switch1 as well as on Switch2.

Now configure LACP on both the switches as below. Configure Active mode on switch1, and passive mode on switch2 as below;

Configure the same setting on Switch2. After configuring the above setting on Switch2, configure LACP on Switch1.

Use the following command to check etherchannel formation;

The above screenshot defines that LACP has been used in it. But, if you want to know how many links have been used in it, then run another command;

It shows that four interfaces has been used in it, and the number of port-channel has been configured which is only 1. It also shows that the port-channel is synchronized and up through the Po1(SU).

LACP Load Balancing

Basically, LACP is the load balancing protocol which distribute traffic over different aggregated links. It uses different hashing algoriths to optimize performance based on traffic patterns such as;

• Source/Destination MAC Address Hashing
• Source/Destination IP address Hashing
• Source/Destination TCP/UDP Port Hashing
• Layer 2, Layer 3, or Layer 4 Hashing Combinations

We are going to configure LACP link aggregation load-balancing on both the switches. However, it is not necessary to configure the same load-balancing method on both the switches. It depends on the traffic patterns and design.

Configure the LACP load-balancing on switch using ‘src-mac’ method;

If we configure the load-balancing on switch1, but we don’t configure it on switch2. Then switch2 may not distribute traffic efficiently. That’s why we must configure load-balancing on the switch2 as well.

Check and confirm load-balancing by the command;

Run the same command on switch2;

What is System Priority in LACP protocol?

The system priority helps us to decide which switch will always initiate LACP packets when both the switches are connected for aggregation. Lowering the priority will make the switch as an active member in LACP link aggregation. The default priority for LACP aggregation protocol is 32768.

We will make switch1 an Active member of the aggregated link by lowering its system-priority to 4096.

After configuring the system-priority, Switch1 will generate lacp initiation process. Verify the system-priority through the following command;

If we run another neighbor command, then it will explore some more information related to LACP configuration.

Switch1 peer has SP flag while Switch2 peer has SA flags, which means Switch1 is in active mode, while Switch2 is in passive mode. However, there are also some other peers as well;

Flags	Meaning
SA	Slow LACPDUs, Active Mode
SP	Slow LACPDUs, Passive Mode
FA	Fast LACPDUs, Active Mode
FP	Fast LACPDUs, Passive Mode