To finish up the first topic under LAN Switching Technologies, identifying enhanced switching technologies. I will give you an overview of Etherchannels.
What they are, what they do and how to configure one for yourself.
Firstly an Etherchannel is a configuration that allows a switch to treat two parallel links as one physical link. This also allows STP to treat the entire channel as one and either block or forward traffic over this link as per vlan.
As a result of this, the switch can now load balance traffic over the channel. Without Etherchannel one of the links would be blocked by STP, as is shown in the example below.
In the above example, SW0 and SW1 are connected via two links. Fa0/1 to Fa0/1 and Fa0/2 to Fa0/2, but due to STP it has blacked Fa0/2 on SW1 from forwarding frames. STP at this point see’s these two links as they are, separate from one another.
When we run a show spanning-tree command on SW1 we discover that, port Fa0/1 is in a forwarding state as the RP, Port Fa0/2 is blocked and Fa0/3 is the DP. Running the show spanning-tree command on SW0 would show us that it is the Root Bridge and all three ports are DP’s or Designated Ports.
To make STP use both the Fa0/1 and Fa0/2 links on SW0 and SW1 we need to configure the interfaces in the same Channel-group or Etherchannel. Doing so will change how STP see’s those two links. From two separate links to one link which can be used to load balance. We are not actually making one physical link but like a VLAN where we can make many Virtual LAN’s we are creating one Virtual Link using 2 parallel links.
For this network the configuration is virtually the same, just remember you will need to add the correct interfaces in the same channel group for an Etherchannel to become active.
The following image shows you how to correctly configure an Etherchannel, using the command channel-group 1 mode on, you can configure upto 8 parallel links on the one Etherchannel.
Below you can see the new show spanning-tree output displayed on SW1 after the Etherchannel has been configured. Notice the Po1, that indicates that all the configuration parameters checked and that is the new channel group 1, which was configured earlier.
The new diagram, with the two links now bundled in the new Etherchannel. Notice how Fa0/2 has come out of blocking mode to become a RP
Posted by Josh Gahan 
