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| 3.9 |  | EtherChannel |
| 3.9.5 |  | Link Aggregation Control Protocol (LACP) |

Now, we change gears and introduce the open-standard equivalent to PAgP, the Link Aggregation Control Protocol (LACP). Defined in IEEE 802.3ad, LACP allows Cisco switches to manage Ethernet channels with non-Cisco devices conforming to the 802.3ad specification. It is likely that LACP will become the norm for Catalyst switches, just as Cisco migrated from ISL to IEEE 802.1Q. (Cisco cannot always wait for the IEEE to complete its standards process before incorporating new features on its devices.) To have LACP handle channeling, use the active and passive channel modes. To start automatic EtherChannel configuration with LACP, configure at least one end of the link to active mode to initiate channeling, because ports in passive mode passively respond to initiation and never initiate the sending of LACP packets. The various LACP modes and their definitions are detailed in Figure .

The on mode forces the port to channel without LACP. With the on mode, a usable EtherChannel exists only when a port group in on mode is connected to another port group in on mode. The Off mode prevents the port from channeling. The Passive mode is similar to the PAgP auto mode, as it places the port into a passive negotiating state, in which the port responds to LACP packets it receives but does not initiate LACP packet negotiation. This is the default.

The parameters used in configuring LACP are as follows:

- System priority – Each switch running LACP must be assigned a system priority. The system priority is used with the switch MAC address to form the system ID. It is also used during negotiation with other systems.
- Port priority – Each port in the switch must be assigned a port priority. The port priority is used with the port number to form the port identifier. The port priority decides which ports should be put in standby mode when a hardware limitation prevents all compatible ports from aggregating.
- Administrative key – Each port in the switch must be assigned an administrative key value. A port’s ability to aggregate with other ports is defined with the administrative key. An administrative key is meaningful only in the context of the switch that allocates it; there is no global significance to administrative key values. A port’s ability to aggregate with other ports is determined by these factors:

> - Port physical characteristics, such as data rate, duplex capability, and point-to-point or shared medium
> - Configuration constraints that can be established

When enabled, LACP always tries to configure the maximum number of compatible ports in a channel, up to the maximum allowed by the hardware (eight ports for Catalyst switches). If LACP cannot aggregate all the ports that are compatible (for example, if the remote system has more restrictive hardware limitations), all the ports that cannot be actively included in the channel are put in hot standby state and are used only if one of the channeled ports fails.