Suppose you are trying to communicate or send data across a network to another PC or server, but the network is congested.  One reason for this is that you might not have sufficient bandwidth.  If you look at the diagram I included at the top of this page, you will notice that there are two Ethernet connections connecting SW1 and SW2.  If only one Ethernet cable were connecting them, it could cause traffic to slow down due to a lack of bandwidth.  If, however, you connect a second Ethernet cable, you can double the bandwidth. 

EtherChannel is a technique that uses a protocol to combine the power of several physical ports and act as a single logical port.  Usually, a protocol called ‘spanning tree’ will block this from happening.  If EtherChannel is enabled, it bypasses Spanning Tree and unblocks any ports that Spanning Tree has blocked. 

The circle in the middle of the diagram indicates an EtherChannel connection.  In this lab, I used a standard Cisco proprietary protocol called PagP or ‘port aggregate protocol’.  There is also an industry-standard protocol called LACP, or ‘link aggregate protocol’.  You first need to specify an interface range and then link them together with the command channel-group 1 mode desirable.  I performed this action on both switches, but you only need to do it on one side if the other side is set to auto. 

The last step I did was to check the new bandwidth.  If you use the command show int F0/2 on SW1, you will see a section labeled ‘BW’.  BW stands for bandwidth, and F0/2 refers to a Fast Ethernet port, which has a bandwidth of 100,000 kilobits or 100 Mbps.  When you configured channel-group 1, the switch created a new interface called po1.  P01 is the EtherChannel link.  You can check the increased bandwidth by looking at the interface using the ‘show int p01’ command.  The interface should have a value of 200,000 Kbit.  The increase is due to P01 now having a combined bandwidth across two interfaces. 

I tested the PCs and they can ping one another.  It’s best practice to change the default load-balancing method on the EtherChannel to use the source and destination MAC address with this command:

port-channel load-balance src-dst-ip

The default setting is to load-balance only based on the source MAC address.  Load-balancing with only the source MAC address can cause traffic from the same device to never be routed across a different line, leading to an unbalanced result.

Here is the link to the interactive lab: georgebatton/Etherchannel: Etherchannel Lab

This explanation should help with a better understanding of EtherChannel.  It’s a fairly simple concept and setting it up is simply a matter of knowing the proper commands.