Cisco CCNA Certification: Etherchannels, Spanning Tree Protocol

By: Chris Bryant

Cisco CCNA certification looks great on your resume, and it also teaches you vital real-world networking skills.  One exam topic that you'll definitely see in today's networks is the configuration of an Etherchannel on a Cisco router.  You need to know how to build and verify one - so let's build one right now!

In this Cisco training tutorial, we'll use two switches that are connected at ports 0/11 and 0/12 via crossover cables.  SW1 is the root bridge, SW2 is the non-root bridge.  Even though there are two separate physical connections, spanning tree protocol (STP) will only allow the use of one, as verified by the partial output of "show spanning vlan 1" on SW2.

Fa0/11           Root FWD 19        128.11   P2p

Fa0/12           Altn   BLK 19        128.12   P2p

It's a waste of network resources not to use that second physical path.  Additionally, there's going to be a delay in cutting over from 0/11 to 0/12 if the forwarding path goes down.  We can get around both of these issues by creating an Etherchannel.  An Etherchannel is a logical bundling of physical paths into one logical path.  By configuring an Etherchannel on both 0/11 and 0/12, we will be left with one logical connection that will actually use the full capacity of both physical connections!

SW1(config)#int fast 0/11

SW1(config-if)#channel-group 1 mode on

Creating a port-channel interface Port-channel 1

The channel-group command must be configured on each physical interface that we want to place into the Etherchannel, so we'll have to configure it on ports 0/11 and 0/12 on both SW1 and SW2.  Naturally, the group number used must match among all ports placed into the Etherchannel.

Did you notice the router message "creating a port-channel interface port-channel 1"?  You can verify the creation of that virtual interface just as you would a physical interface:

SW1#show int port-channel 1

Port-channel1 is up, line protocol is up (connected)

  Hardware is EtherChannel, address is 000f.90e1.c24b (bia 000f.90e1.c24b)

Note the readout "Hardware is EtherChannel".  Also, where we once saw two physical ports when we ran "show spanning vlan 1", we now see only one - the port-channel.  Also note that where the individual ports had a port cost of 19, the Etherchannel has a cost of 12 due to its increased bandwidth capabilities.

Po1              Desg FWD 12        128.65   P2p

Etherchannels give us a performance benefit and a fault tolerance benefit as well.  Performance is increased since all physical channels between the two switches can be used, rather than the one STP allows by default.  Fault tolerance is also increased, since a physical channel bundled into the Etherchannel can go down without an STP recalculation -- STP sees the Etherchannel as one single connection!  

Etherchannels aren't just important to know about for the CCNA examBusiness Management Articles, they're commonly used in real-world networks as well - so make sure you know how to configure and troubleshoot them!

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