SWITCH 300-115 1.5 Configure and verify EtherChannels

1.5.a LACP, PAgP, manual
Etherchannel comes in three flavors, on, my preference often called static or manual,  whereby there is no negotiation of the channel and thus, no extra protocol traffic, PAGP or port aggregation protocol which is Cisco proprietary, and LACP or link aggregation protocol which is the open standard 802.3ad. It is interesting to note that PAGP is not supported on Cisco’s Nexus OS line and that LACP is the preferred method of aggregation from the data center side of the house.
They all have one thing in common, however, and that is to bundle together a group of ports on a switch with the net effect of increasing bandwidth between two connected switches. For instance switches will support 2 to  8 active members on each side of a connection; so considering they are 100 Meg each, a total of 1600 Meg bandwidth can be achieved between them.

Another 8 ports may be used as backup but only 8 may be active at one time. The aggregated ports of a channel must be setup on the individual switch but the ports do not have to be contiguous, and they can cross modules in the event of a chassis type switch or a stack. Also the channel numbers on either side do not have to match, but I advise making it a practice of matching the sides as it makes for easier documentation, and more intuitive troubleshooting.

Another inherent benefit is lessening the impact of Spanning-tree on the network. The etherchannel or port-channel is treated as a single link by STP, therefore there can be no blocking of individual links within the channel, although multiple redundant channels between switches would still be governed by spanning-tree.
A caveat in the creation of a channel is that both sides port configurations need to be exactly the same or the channel will not form. This is one reason why care should be taken when using on or manual mode because without negotiation, there will be no warning in the event of misconfiguration. Another thing to keep in mind is that there is no mixing of channel protocols. For instance, PAGP can use desirable/auto, desirable/desirable, but not auto/auto, similar to DTP. LACP can be act/passive, active/active but not passive/passive. On mode is simply on for both sides.