Link aggregation is the capability to produce one logical link using several physical links between two devices. Network devices and management functions care for the link aggregation group (LAG) of multiple Ethernet connections as a single link.
Link aggregation allows load sharing among the multiple physical links, rather than having STP block one or more of the links. EtherChannel is the type of link aggregation used in switched networks.
We can configure EtherChannel either manually or dynamically. The dynamic method of EtherChannel configuration is the Cisco-proprietary Port Aggregation Protocol (PAgP) and the IEEE 802.3ad-defined protocol Link Aggregation Control Protocol (LACP).
Usually, the link speed between switches maybe 10 Mbps, 100 Mbps, 1000 Mbps and 10,000 Mbps. Normally if the redundant physical links are established between the two switches, the spanning-tree protocol block all redundant path to the same switch accept one. It may be possible to use faster links, such as 10 10,000 Mbps.
Adding faster links between access and distribution links are very expensive. As the speed on the access links increases, the fastest possible port on the aggregated link is not enough to aggregate the traffic coming from all access links.
The figure-1 illustrates the access and core layer with STP. We know that STP is enabled by default on switch devices. STP will block redundant links to prevent routing loops.
Traffic coming from several links usually 10/100/1000 Mb/s aggregates on the access switch and must be sent to distribution switches. So a link with higher bandwidth must be available between the access and distribution switches. And with the STP may be the higher link is not enough for the incoming traffic.
So, how can we increase the speed between the core and access layer or any other switches? We can multiply the number of physical links between the switches to increase the overall speed of switch-to-switch communication. The method used to multiply the speed of several numbers of links together is called EtherChannel. Figure-2 illustrates the Ether channel.