CCNA Certification Training: Etherchannels And STP
By Chris Bryant, CCIE #12933
A common topic on Cisco exams as well as a common technology in production networks is the Etherchannel. In today's Cisco training article, we'll build an Etherchannel and make sure it's working correctly.
Before we get started, I want to mention that well over 250 free Cisco videos, practice exams, and fully-illustrated tutorials are waiting for you on my Cisco certification tutorial page - so head over there when you're done here!
An Etherchannel may sound complicated, but it's really not. All we're doing is taking multiple physical connections between two Cisco switches and logically bundling them. Let's take a look at the setup we'll use today, and then discuss why we'd create an Etherchannel.
SW1 and SW2 are connected by two crossover cables, one cable connecting port fast0/11 on both switches and the other connecting fast0/12 on both switches. I've created VLAN 23 for this example, and show spanning vlan 23 on both switches verifies that SW1 is the root bridge for that VLAN.
On SW2, one of the two ports has been placed into blocking mode. STP will only allow one of the two paths between SW1 and SW2 to be used; this is to prevent switching loops. That's a good thing, but it does waste bandwidth and network resources to only use one of these two available paths. Also, should the forwarding path go down, the cutover to the currently blocked path is not immediate.
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Configuring an Etherchannel allows us to use both paths while avoiding switching loops. How? STP will see the Etherchannel as a single path, even though the Etherchannel consists of multiple physical channels. Let's create the Etherchannel with the channel-group command and see how STP reacts.
A new interface, port-channel 1, has been created. Like a loopback interface, the port-channel interface is a logical interface - it doesn't physically exist. The port-channel interface represents the logical bundle of physical interfaces that make up the Etherchannel.
How does STP see the Etherchannel?
As a single connection! Both physical channels are now in use. Note the cost of the Etherchannel is 12 where the cost of the individual ports was 19. That reflects the increased bandwidth available due to the bundling.
Etherchannels are commonly used in today's networks, and I'd be surprised if you didn't get a question or two about them on your CCNA exam. When you know how to configure an EC and what exactly is going on when you do so, you're on your way to success in both the network room and the exam room!
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To your success,
Chris Bryant
CCIE #12933
chris@thebryantadvantage.com
I'm Paying It Forward Bigger Than Ever Before.
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