Static aggregation and the need for LACP
Link aggregation combines several physical connections into one logical interface. It can be configured statically or through a negotiation protocol. In a static configuration, an administrator assigns ports on both ends to the same group, for example ports 1, 2, 3, and 4. This mode is often labeled "on."
Static assignment has several limitations:
- Configuration errors: Operation depends on matching settings at both ends. A wrong port, a cable connected to another interface, or different group parameters can leave a port outside the intended channel or cause frame loss.
- Limited diagnostics:A static LAG exchanges no partner information. When settings differ, the administrator checks cabling, port state, and configuration on both devices because "on" mode does not identify which condition failed.
- No partner-state exchange: Static mode does not negotiate with the far end. The local physical layer can detect a broken cable, while a remote port failure that leaves the local interface up can cause the device to continue sending traffic toward that link.
LACP adds an exchange of state between the devices. Ports can be checked, admitted to the group, or removed after the partner state expires. The aggregation state machine keeps the member set separate from ordinary port forwarding, so a port with an incompatible partner state can remain physically up without carrying LAG traffic. This distinction is useful when diagnosing a channel that has fewer active members than its configuration lists.
What is LACP? The Link Aggregation Control Protocol
The Link Aggregation Control Protocol (LACP) is an IEEE mechanism first defined in 802.3ad and moved into 802.1AX. It dynamically assigns compatible Ethernet ports to one logical channel, called a Link Aggregation Group (LAG). IEEE 802.1AX-2020 is the current standard for this function.
LACP lets two connected devices, such as switches, exchange information about their ports. The control frames identify the device, port, operational key, and aggregation state. Each side reports its own actor information and the partner information it currently knows. The devices use this information to select ports that meet the group conditions and to set their frame collection and distribution states. The protocol limits some effects of mismatched settings, but it does not replace correct configuration or loop-prevention protocols.
LACP negotiation in active and passive modes
To establish a LAG, ports on both devices must be configured to participate in LACP. The protocol defines two port activity modes:
Active Mode
A port in Active mode initiates LACP negotiation. It periodically sends control frames called to its partner to report its state and begin LAG selection.
Passive Mode
A port in Passive mode does not start negotiation. It waits passively for an LACPDU from its partner. After receiving one, it responds and can join a LAG if the remaining parameters match. Without an active partner it remains an independent port.
A LAG can form when the activity modes create a compatible pair and the remaining port parameters match:
| Switch 1 Port Mode | Switch 2 Port Mode | Result |
|---|---|---|
| Active | Active | LAG can form when parameters match. |
| Active | Passive | LAG can form when parameters match. |
| Passive | Active | LAG can form when parameters match. |
| Passive | Passive | No LAG is formed. (Neither port sends an LACPDU) |
At least one side must use Active mode to start the LACPDU exchange.
LACPDUs and port state information
LACP negotiation uses an exchange of LACPDU frames. Each frame carries information needed to identify the partner, evaluate port compatibility, and maintain the port state in a LAG. The main fields include:
- System Information:
- System priority: A configurable value used when selecting the system that controls aggregation. The lower value has precedence, and the MAC address breaks a tie.
- System MAC address: The device's MAC address. The priority and MAC address together form the System ID.
- Port Information:
- Port priority: A configurable value used when selecting ports if the number of compatible group members is limited.
- Port number: The number of the port sending the LACPDU. The priority and port number together form the Port ID.
- Operational key: A value that represents the parameters used by an implementation to group compatible ports. Ports with different keys are not candidates for the same aggregator. The method used to calculate the key depends on the implementation and configuration.
- State flags: Fields describing activity, aggregability, synchronization, frame collection and distribution, and expiry of partner information.
The devices compare actor and partner information. A port can be selected for a LAG when its state is compatible with the partner, its operational key permits common grouping, and the device can move it into collecting and distributing states. A port can therefore be physically active while remaining outside the usable aggregator. The System ID identifies the negotiation endpoints, but the two identifiers are not expected to be equal.
Effects of using LACP
LACP adds partner-state control, compatible-port selection, and handling of changes in the group to static aggregation. The response depends on device configuration and timer settings.
Partner configuration checks
LACP compares partner information. If a cable reaches a port from another group or the port parameters differ, the device can leave the interface outside the LAG collecting and distributing state. LACP does not replace STP, so loop protection must be evaluated together with the switch configuration.
Detecting partner-state loss
Devices send LACPDUs periodically. The interval depends on the selected mode and implementation. Cisco documentation describes 30 seconds for the normal mode and 1 second for the fast mode. Missing frames for the expiry period can remove a port from the active aggregator even when the local interface remains up. The device can then send new frames through the remaining members, while the reaction time depends on timers and platform behavior.
Handling group changes
After a new port is configured at both ends, LACP can detect its partner, compare the parameters, and add the port to the active group. If the port stops meeting the conditions, its aggregation state can be withdrawn without rebuilding the entire channel configuration.
Interoperation between vendors
LACP is an IEEE standard, so it can connect equipment from different vendors when both sides implement a compatible version of 802.1AX. Software can still impose its own limits for member count, timers, and supported parameters. The standard does not require one vendor across the channel.