As I promised last time, I will be providing more updates regarding the differences between the previous CCNA track and the new CCNA Routing and Switching track announced by Cisco on March 26th. One of the major differences is that the revised track will include lecture and lab on the topic of Etherchannel, or port aggregation. The prior version barely mentioned this feature of the Cisco IOS. Let’s take a moment to review this topic.
Here is a picture representing the idea of etherchannel:
Here we have two switches connected with two FastEthernet links. In the absence of etherchannel, these switches would view these as two separate links which would mean Spanning-Tree would block one of the links. This is nice for loop prevention but not our intended goal. Since these are in actuality two physical links (and one of them would end up blocked) we run the risk of oversaturating the physical link if we are attempting to share too much data between these switches. Enter the solution – etherchannel. The concept is that the two (or more – up to 8 in all) physical links are bundled into one logical link. This is also called aggregating links, bundling links, or creating a port channel. It is best to bundle physical links in powers of two; that is, 2, 4 or 8 ports in the channel.
How does this benefit us as network admins? I now have higher bandwidth at my disposal. For example, if the two links shown above are both 100Mbps, then I have an effective throughput of 200 Mbps. Note: This does not mean that if I had a PC connected to each switch, that I could transfer a file at 200 Mbps. In an etherchannel solution, one can never exceed the actual speed of any one of the physical links for any individual session. Overall, I would have an effective throughput of 200 Mbps, which would mean if I had multiple endpoints all sharing the logical link, the overall bandwidth would be the 200 Mbps.
Etherchannel also provides redundancy in the sense that if one of the physical links dies, the port channel itself continues to function without user intervention. As long as at least one of the physical links is active, the etherchannel is active as well.
We do have choices when we configure etherchannel also. There are two protocols from which to choose – PAgP and LACP. PAgP is a Cisco –proprietary protocol used to bundle multiple physical links into one logical link. It works very well when you are running all Cisco gear. For shops using other vendors, LACP will perform almost identical functionality. Cisco devices can run both of these protocols. While I won’t get into the actual configuration commands, I will mention your options for both of these protocols as far as successful establishment of the channel. Both protocols have three choices: Hard code the channel with no negotiation, willing to negotiate the channel including initiating the channel, or being willing to channel but not to initiate the channel. The terminology for each protocol is different for these actions, but the results are the same. If you are running PAgP, the key words are ON, AUTO, and DESIRABLE. For LACP, the keywords are ON, PASSIVE, and ACTIVE.
Here is a table showing the outcome if the above choices are made:
This is just one of the adjustments that new CCNA R&S candidates will have to learn. There are more. I will continue to share tidbits of the new track in future blogs.