What are the three steps to configure an LACP EtherChannel on a Cisco switch?

Select the interfaces, assign them to a channel-group with a mode, then configure the port channel interface.

* Three-step LACP EtherChannel configuration. *

Step 1: Select the physical interfaces:

S1(config)# interface range FastEthernet 0/1 - 2

Step 2: Create the port channel and set the LACP (Link Aggregation Control Protocol) mode:

S1(config-if-range)# channel-group 1 mode active

This creates the logical Port-channel 1 interface automatically.

Step 3: Configure Layer 2 settings on the port channel:

S1(config-if-range)# exit
S1(config)# interface port-channel 1
S1(config-if)# switchport mode trunk
S1(config-if)# switchport trunk allowed vlan 1,2,20

Key details:

• The channel-group number is local to the switch — it doesn't have to match the other side (but the protocol mode must be compatible)
• Use mode active for LACP, mode desirable for PAgP (Port Aggregation Protocol), or mode on for static
• The interface port-channel command enters the logical interface for L2 configuration
• Settings applied here propagate to all member interfaces

Go deeper:

• Cisco Switch EtherChannel Configuration (Networks Training) — channel-group mode active, the port-channel interface, and trunk config on both switches.
• How to Configure EtherChannel (PAgP & LACP) — video walkthrough of LACP and PAgP configuration.