Increase network bandwidth using Cisco's EtherChannel

David Davis explains how to configure EtherChannel to increase your bandwidth and provide redundancy on a Cisco network.

In my previous article on the Virtual Router Redundancy Protocol (VRRP), I wrote that VRRP is a very efficient way of load balancing and achieving redundancy on routers. EtherChannel technology is another way for you to increase the bandwidth of your core network links and provide redundancy of LAN links.

What is EtherChannel?

EtherChannel allows you to achieve greater speed by bundling Fast Ethernet or Gigabit Ethernet links, which makes a switch or router use the merged ports as a single port. This gives you the most bang for your buck from your port cost. EtherChannel uses a Cisco-proprietary hashing algorithm to achieve this link bonding.

EtherChannel can be configured on many different switch platforms such as the 6500/6000, 5500/5000, 4500/4000, and 2950/2960 series. In this article, I'm going to focus on how to configure EtherChannel on the 2950/2960 line of switches because those are some of the most deployed and well-known platforms.

There are a few basic things that you should know about EtherChannel:

  • All interfaces using EtherChannel must be set up with the same speeds and duplex modes.
  • Specifically for the Catalyst 2950 switches, the EtherChannels are limited to six with eight ports per EtherChannel, and all must have the same speed and be configured on Layer 2 interfaces.
  • While it might seem obvious, don't forget that the interfaces must be enabled. If not, the switch thinks that there is a problem and will use another interface path unnecessarily.

As you probably remember, the Datalink layer has a Frame Check Sequence (FCS) that adds a preamble (a series of alternating 0's and 1's) along with other information, to the front of a data packet and inserts it into the destination and source address. EtherChannel balances the traffic on your network by reducing part of the binary pattern formed from the addresses in the frame, which is in the Network layer (Layer 2 - DataLink layer), to a numerical value. And since your Fast Ethernet links are now combined into one channel, it allows for redundancy, in case one of the links goes down.

How do you configure EtherChannel?

Let's look at a sample of a typical EtherChannel configuration using load distribution and forwarding methods.

Figure A

Graphic Courtesy of Cisco Systems
As you can see in Figure A, there are switched links on the 10/100 Ethernet interfaces combined into one EtherChannel for the Cisco 2950 switch. To configure EtherChannel, you must also understand a little behind the scenes of Port Aggregation Protocol (PAgP). This protocol assists the auto-creation of EtherChannels, learning the similarities and capabilities of each of your interfaces before grouping them together into a logical link. This is a great efficiency for your network.

There are options for PAgP, like auto or desirable, which unconditionally enables PAgP.

Let's look at how you would configure EtherChannel on a Gigabit Ethernet interface 0/1 and 0/2 with PAgP mode desirable.

switch1(config)# interface range gigabit ethernet0/1 -2
switch1(config-if# channel group 5 mode desirable

Of course, you would always want to see the results of your configuration. The following command shows you the EtherChannel information in a brief, detailed, and one-line summary form.

Switch1# show etherchannel [channel-group-number]{brief | detail | load balance | port | port-channel | summary}

Benefits of using EtherChannel

  • Increased Bandwidth: Use EtherChannel and combine two or four links into one logical link. It will double or quadruple your bandwidth. For example, four 100Mb Fast Ethernet connections bonded into one could provide you up to 800Mb/second, full duplex.
  • Provides Redundancy: Since there are many Ethernet links combined into one logical channel, it automatically allows more available links in case one or more links go down.
  • Load Balance Traffic: EtherChannel balances the traffic load across the links, thereby increasing efficiency on your networks.

To learn more about EtherChannel, please see the official Cisco documentation: Understanding EtherChannel Load Balancing and Redundancy on Catalyst Switches and the Cisco 2950 Switch Configuration Guide - Configuring the Switch Ports.

David Davis has worked in the IT industry for 15+ years and holds several certifications, including CCIE, CCNA, CCNP, MCSE, CISSP, VCP. He has authored hundreds of articles and numerous IT training videos. Today, David is the Director of Infrastructure at Train Train Signal, Inc. is the global leader in video training for IT professionals and end users.

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How would you compare and contrast the channel-group mode being set to "on" vs. "desirable" I'm wondering.


I did some research with Cisco prior to creating an EtherChannel because we had a user that wanted 4 1Gb Ethernet connections to their server. On a Cisco 65x with 6548 modules (which are capable of 10, 100, 1000Mb) I was told there is a 1Gb ASIC per 8 interfaces. So setting up an EtherChannel within the same 8 ports doesn't get you any more bandwidth than the 1Gb because the ASIC is the limitation, plus any other systems connected within that group of 8 interfaces can become starved for bandwidth. Separating the interfaces so that no two interfaces are in the same 8 port group doesn't help either as the EtherChannel is still controlled by a single 1Gb ASIC and again the other seven systems on that ASIC can become starved for bandwidth. Right now I do not recall if separating the interfaces across different modules provided the increased bandwidth or not. (Maybe the distributed line cards would) The user has a single 1Gb interface into their server and we are monitoring the utilization of this interface. If we find that they are pushing the limits of the interface we will then look into what modules can support simultaneous multiple 1Gb connections or 10Gb interfaces.

Duluth Networker
Duluth Networker

"On" uses specific interfaces, forcing them to trunk together. Good for static environments where you're not making changes or needing to scale. "Desireable" means that ports can join the EtherChannnel automatically when the conditions on both ends are met--makes adding more ports easier because they come up automatically in the trunked EtherChannel, instead of needing to be added manually.

Duluth Networker
Duluth Networker

Five years ago we moved away from EtherChannel configurations, replacing that gear with Nortel's Multi Link Trunks. I found the MLT's to be much more intuitive and easy to configure than the EtherChannel links. The load balancing was automatic, and the active-active links that connect an edge switch to two core switches failover very quickly--you might lose a ping, you might not. MLT's scale to 8 per edge devices in our environment, and are split going to two core switches, for eight 1-Gb non-blocking throughput. We span them across multiple blades on each chassis switch for diversity (Nortel calls it DMLT, or Diverse Multi Link Trunking)--if a blade should ever fail, no users are affected.

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