EtherChannel is the Cisco term for the technology that enables the bonding of up to eight physical Ethernet links into a single logical link. EtherChannel was originally called Fast EtherChannel (FEC), as it was only available on Fast Ethernet at the time.
With EtherChannel, the single logical link's speed is equal to the aggregate of the speeds of all the physical links used. For example, if you were to create an EtherChannel out of four 100-Mbps Ethernet links, the EtherChannel would have a speed of 400 Mbps.
This sounds great, and it is, but the idea is not without problems. For one thing, the bandwidth is not truly the aggregate of the physical link speeds in all situations. For example, on an EtherChannel composed of four 1-Gbps links, each conversation will still be limited to 1 Gbps by default.
The default behavior is to assign one of the physical links to each packet that traverses the EtherChannel, based on the packet's destination MAC address. This means that if one workstation talks to one server over an EtherChannel, only one of the physical links will be used. In fact, all of the traffic destined for that server will traverse a single physical link in the EtherChannel. This means that a single user will only ever get 1 Gbps from the EtherChannel at a time. (This behavior can be changed to send each packet over a different physical link, but as we'll see, there are limits to how well this works for applications like VoIP.) The benefit arises when ...