Basically, you should configure the logical interface and then put the physical interfaces into the channel group: 

(config)# interface port‐channel number 

For a Layer 3 EtherChannel, add the following: (config‐if)# no switchport

(config‐if)# ip address address mask 

Then, at each port that is part of the EtherChannel, use the following:

(config)# interface { number | range interface – interface}

(config‐if)# channel‐group number mode {auto | desirable | on}  

Putting the IP address on the Port Channel interface creates a Layer 3 EtherChannel. Simply putting interfaces into a channel group creates a Layer 2 EtherChannel, and the logical interface is automatically created. 

The Cisco proprietary Port Aggregation Protocol (PAgP) dynamically negotiates the formation of a channel. There are three PAgP modes: 

■ On: The port channels without using PAgP negotiation. The port on the other side must also be set to On. 

■ Auto: Responds to PAgP messages but does not initiate them. Port channels if the port on the other end is set to Desirable. This is the default mode. 

■ Desirable: Port actively negotiates channeling status with the interface on the other end of the link. Port channels if the other side is Auto or Desirable. 

Link Aggregation Control Protocol (LACP) is an IEEE standard protocol, IEEE 802.3ad, which does the same thing. LACP modes follow: 

■ On: The port channels without using LACP negotiation. The port on the other side must also be set to On. 

■ Active: Port actively negotiates channeling with the port on the other end of the link. A channel forms if the other side is Passive or Active. 

■ Passive: Responds to LACP messages but does not initiate them. A channel forms only if the other end is set to Active.

Verifying an EtherChannel 

Some typical commands for verifying include the following: #  show running‐config interface number 

#  show interfaces number etherchannel 

#  show etherchannel number port‐channel 

#  show etherchannel summary 

#  show etherchannel load‐balance 

References:

http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/release/12.1_13_ea1/configuration/guide/swethchl.html

http://www.dummies.com/how-to/content/etherchannel-configuration.html