There are several LACP parameters that are contained in the LACP PDUs that are exchanged between switches. After exchanging LACP PDUs (also referred to as LACPDUs in some texts), the actor (local switch) and the partner (remote switch) come to agreement about each other’s settings. The switches can now decide whether the ports at each end of the link can be added to an aggregation. LACP uses the following parameters:
- LACP System Priority
- LACP Port Priority
- LACP Administrative Key
These three LACP parameters, which are illustrated in Figure 5-8 below, will be described in detail in this section:
Fig. 5-8. LACP PDU (LACPDU) Parameters
LACP System Priority
You must configure an LACP system priority on each device running LACP. It is important to know that you must also configure an LACP system priority at each end in order for LACP to successfully negotiate the EtherChannel group between the two endpoints. The LACP system priority can be configured automatically or through the Command Line Interface (CLI). LACP uses the system priority with the device MAC address to form the system ID as well as during negotiation with other systems. This is illustrated below in Figure 5-9:
Fig. 5-9. Deriving the LACP System ID from the MAC Address and System Priority
LACP Port Priority
As is the case with the LACP system priority, you must also configure an LACP port priority on each port configured to use LACP. The port priority can be configured automatically or through the CLI. LACP uses the port priority to decide which ports should be put into standby mode when there is a hardware limitation that prevents all compatible ports from aggregating. In other words, if more than eight links are assigned to an EtherChannel bundle running LACP, the protocol uses the port priority to determine which ports are placed into a standby mode (i.e. placed into the EtherChannel) if one or more of the current active LACP links fails. LACP also uses the port priority with the port number to form the port identifier. This concept is illustrated below in Figure 5-10:
Fig. 5-10. Deriving the LACP Port ID from the Port Priority and Port Number
LACP Administrative Key
LACP automatically configures an administrative key value on each port configured to use LACP. The administrative key defines the ability of a port to aggregate with other ports. Only ports that have the same administrative key are allowed to be aggregated into the same port channel group. This is illustrated below in Figure 5-11:
Fig. 5-11. Aggregating LACP Ports Based on the Administrative Key
A port’s ability to aggregate with other ports is determined by physical characteristics, such as data rate, duplex capability, and point-to-point or shared medium, or by administrator-defined configuration restrictions or constraints.