Etherchannal

Link Aggregation allows the grouping of up to 8 physical ports into one link.all port in the group must be the same, speed, dup, access (same vlan) or trunk (same encap, vlans allow).

• PAgP, is cisco and has three port modes, auto and desirable, and on.
• If two link are in desirable they will not negotiate a link
• If connecting to servers which do not use PAgP then use silent  
•  LACP, is not cisco and has two modes, active and passive
• if two links are in Passive they will not negotiate a link
• ON, uses no negotiation.
• Must be set on all

Is configure by first creating a logical interface called a "port-channel" with a group id

• Switch(config)#interface port-channel 1

Then place the physical interfaces in that group and set its mode depending on the protocol used, and then set the protocol

• Switch(config-if)#channel-group 1 mode on\des\auto\act\pass
• Switch(config-if)#channel-protocol pagp\lacp

Then you can configure the port-channel interface like a normal interface with trunking if needed.

• Switch(config)#int port-channel 1
• Switch(config-if)#int mode trunk

There can also be a layer 3 interface port-channel by adding an IP address as well.
Also uses a load balancing method