LACP
LACP uses LACPDUs to exchange aggregation information between LACP-enabled devices.
Each member port in an LACP-enabled aggregation group exchanges information with its peer. When a member port receives an LACPDU, it compares the received information with information received on the other member ports. In this way, the two systems reach an agreement on which ports are placed in the Selected state.
LACP functions
LACP offers basic LACP functions and extended LACP functions, as described in Table 3.
Table 3: Basic and extended LACP functions
Category | Description |
|---|---|
Basic LACP functions | Implemented through the basic LACPDU fields, including the system LACP priority, system MAC address, port priority, port number, and operational key. |
Extended LACP functions | Implemented by extending the LACPDU with new TLV fields. This is how the LACP MAD mechanism of the IRF feature is implemented. The switch series can participate in LACP MAD as either an IRF member device or an intermediate device. For more information about IRF and the LACP MAD mechanism, see IRF Configuration Guide. |
LACP operating modes
LACP can operate in active mode or passive mode.
When LACP is operating in passive mode on a local member port and its peer port, both ports cannot send LACPDUs. When LACP is operating in active mode on the port on either end of a link, both ports can send LACPDUs.
LACP priorities
LACP priorities include system LACP priority and port priority, as described in Table 4. The smaller the priority value, the higher the priority.
Table 4: LACP priorities
Type | Description |
|---|---|
System LACP priority | Used by two peer devices (or systems) to determine which one is superior in link aggregation. In dynamic link aggregation, the system that has higher system LACP priority sets the Selected state of member ports on its side. The system that has lower priority sets port state accordingly. |
Port priority | Determines the likelihood of a member port to be selected on a system. A port with a higher port priority is more likely to become Selected. |
LACP timeout interval
The LACP timeout interval specifies how long a member port waits to receive LACPDUs from the peer port. If a local member port does not receive LACPDUs from the peer within the LACP timeout interval, the member port considers the peer as failed.
The LACP timeout interval also determines the LACPDU sending rate of the peer. LACP timeout intervals include the following types:
Short timeout interval—3 seconds. If you use the short timeout interval, the peer sends one LACPDU per second.
Long timeout interval—90 seconds. If you use the long timeout interval, the peer sends one LACPDU every 30 seconds.