Link Aggregation Control Protocol
An IEEE standard protocol called the Link Aggregation Control Protocol (LACP) (originally 802.3ad, currently 802.1AX) allows network devices to merge several physical network lines into a single logical link, sometimes referred to as a port trunk, port channel, EtherChannel, or Link Aggregation Group (LAG). Through load balancing, redundancy, and greater bandwidth, this procedure improves network performance and dependability.
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Purpose and Goals of LACP
LACP accomplishes a number of important network management objectives:
Increased Bandwidth: LACP improves total data throughput by combining several physical links into a single, faster logical link. For instance, two 1 Gbps lines can be combined to provide a logical link with a maximum capacity of 2 Gbps.
Redundancy and High Availability: In order to prevent a total network outage and guarantee uninterrupted network operation, LACP automatically reroutes traffic to the remaining operational links if one physical link within the LAG fails. Single sites of failure are less likely as a result.
Load Balancing: By distributing network traffic among the aggregation group’s several physical links, LACP helps keep any one link from becoming a bottleneck. It is possible to configure load balancing techniques according to parameters like source/destination MAC addresses, IP addresses, or port numbers.
Automatic Configuration: LACP reduces human configuration errors and dynamically manages connection additions and deletions without requiring user input by automating the aggregate link’s configuration and upkeep.
Enhanced Fault Detection: In addition to identifying link disconnections, LACP is also capable of identifying additional defects, such as faulty link connections, helping to guarantee that traffic is only routed over appropriately aggregated lines.
How LACP Works
LACP uses a dynamic process of monitoring and negotiation to function:
Negotiation: LACP-enabled devices on both ends of the link exchange Link Aggregation Control Protocol Data Units (LACPDUs) during negotiation. The device’s MAC address, operating key, port state, and LACP system priority are all included in these packets. During the detection time, LACP packets are transmitted every second.
Agreement and Aggregation: Devices confirm compatibility and decide to create a LAG based on the information that has been shared. Complementary ports usually share the same operational aggregation key, speed, duplex settings, and Virtual Local Area Network configuration. Following a consensus, the physical connections are combined into a single logical channel.
Link Selection (Actor/Partner): Devices choose an “Actor” during negotiation by first determining the LACP system priority (a higher priority corresponds to a lower numerical value) and then the MAC address (a smaller MAC address if priorities are equal). After that, active links are chosen for the aggregation group based on the Actor’s LACP interface priorities. Smaller interface numbers are selected as active if interface priorities are equal. LACP divides the load among active lines that are suitable to transport traffic.
Monitoring: Within the LAG, LACP keeps an eye on each physical link’s condition and health. It employs a keep-alive mechanism with either slow (30 seconds by default) or quick (1 second) intervals.
Failover: If a link malfunctions (for example, goes down, experiences an OAM fault, or is detected by LACP), LACP immediately identifies the malfunctioning link, eliminates it from the aggregation group, and reroutes traffic to the other active links. LACP can add a failing connection back to the group once it recovers. When in M: N mode, the actual bandwidth is maintained since backup connections are prepared to take over if an active link fails.
Key Features and Benefits
Standardized: A vendor-neutral protocol, LACP is an IEEE standard (802.3ad, subsequently 802.1AX). In contrast to proprietary protocols like Cisco’s Port Aggregation Protocol (PAgP), this enables devices from other manufacturers to construct dynamic LAGs.
Dynamic Management: Without requiring human intervention, LACP enables links to be added and removed from the bundle dynamically.
Reliability: By redirecting traffic in the event of a link failure, reliability offers resilience.
Scalability: Enables the gradual expansion of network backbone speed through the use of more cabling or fiber optic pairs.
Cost-Effective: More port density and bandwidth can be added for less money than if you were to upgrade to faster single links.
Less Network Address Pool Drain: By allocating a single IP address to the entire aggregation process, management is made easier.
Maximum Links: LACP allows a channel to have up to 16 links, of which up to 8 are active at any given time, with the other links in standby mode.
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LACP Modes of Operation
Certain modes can be set up on LACP-participating interfaces to negotiate EtherChannel formation:
Active Mode: When in active mode, an interface sends LACPDUs to the partner device to start LACP discussions. If both sides are in active mode, or if one side is active and the other is passive, an LACP EtherChannel will form.
Passive Mode: In passive mode, an interface does not start the negotiation itself; instead, it reacts to LACPDUs that it gets from a partner. One side being active and the other passive will result in the formation of an LACP EtherChannel. An EtherChannel will not be created if both switches are set up in passive mode.
On Mode: This mode creates the EtherChannel unconditionally by forcing the interface to channel without any LACP negotiation. When a Link Aggregation Group (LAG) is linked to a Wireless LAN Controller (WLC), this mode is utilized because WLCs generally do not implement PAgP or LACP negotiation protocols.
Key Terms
LACP Data Units (LACPDUs): Devices with LACP support exchange packets known as LACP Data Units (LACPDUs) in order to talk and work out link aggregation.
Actor: The gadget that transmits the LACP PDU.
Partner: The gadget getting the LACP PDU.
System ID: A MAC address and LACP system priority combined to create a unique identity for a system.
Aggregation Key: The value that indicates which ports can be combined together is known as the aggregation key. Potential aggregation ports are those that share the same operational aggregation key.
Port State: The term “Port State” refers to a port’s status as active, standby, or incompatible.
LACP is a common and efficient technique for improving network performance and dependability in enterprise networks and data centers since it is extensively supported by network switches and servers. For instance, Cisco routers from the 10000 series can bundle Gigabit Ethernet lines using LACP; however they can have some limitations, such as a maximum of four bundled ports per channel.
LACP vs PAgP
| Feature | LACP (Link Aggregation Control Protocol) | PAgP (Port Aggregation Protocol) |
|---|---|---|
| Standard / Proprietary | IEEE 802.3ad (Open Standard) | Cisco Proprietary |
| Compatibility | Works across different vendors’ devices | Works only on Cisco devices |
| Purpose | Groups multiple links into one logical link | Same purpose: groups links (EtherChannel) |
| Modes | – Active (sends LACP packets)- Passive (waits to receive) | – Desirable (actively negotiates)- Auto (waits for negotiation) |
| Maximum Links | 16 (8 active + 8 standby) | 8 (all active) |
| Failure Handling | Can automatically disable failed links and use standby links | Removes failed links, no standby support |
| Usage | Widely used in multi-vendor networks | Limited to Cisco-only environments |
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