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Etherchannel is a Cisco term that allows a selection of ports to be grouped into a single virtual connection.

PROS

1. Able to move more data than a single dedicated line.
2. Provides Redundancy & Instantaneous failover if a line/port does down.
3. The switch views the virtual connect essentially just like any other line. ie Protocols like STP and VTP work over it.

CONS

1. More Ports used on the switch may have a great cost associated with it.
2. 4x100Mb/sec lines means there is still only a max of 100Mb/sec for each individual conversation.
3. All Ports have to be Identical (Ie. All FastEthernet or All Gigabit)
4. All ports have to be configured the same (ie. Duplex, Speed etc)

Setting up Etherchannel

There are 3 main steps when setting up Etherchannel

1. Specify the ports needed
2. Specify Etherchannel Protocol. There are 2 main protocols:
   • a.) IEEE – LACP
   • b.) Cisco – PAgP
3. Specify the Channel Group ID and Mode. This is done in the same command.
   • a.) Because you can have multiple Etherchannels per switch you will need to specify an ID for each one
   • b.) Depending on if you choose LACP or PAgP you will need to specify different modes.
4. Turn the Ports into trunking mode
5. Because Ports are in trunking mode please also turn on encapsulation ie. dot1q

Commands to setup Etherchannel

Here’s a screenshot from Live Cisco 2960 Switches being setup with Etherchannel:

Switch1

Switch2

What do the happens when something goes wrong with one of the lines with the etherchannel?

Switch2 – Unplug Port fa0/22, wait a few minutes and then plug the port back in:

Let’s go over the last image.

1. Unplug cable.
2. Look at how fast the etherchannel reports the issue. It’s actually BEFORE the port actually goes down!
3. Port goes down
4. I hit <enter> a couple of times to break up the output.
5. Plug the cable back into port fa0/22
6. Interface comes back up
7. Etherchannel rebuilds itself

Main commands to troubleshoot:

show etherchannel summary

• Displays info on status of group, port-channel, protocol and port

show etherchannel port-channel

• Displays configured properties of port-channel, port state, protocol, port security, fast switchover and load share. It also lists the ports and timing info.

show interface trunk

• Displays info about all the trunk interfaces.

Are you getting the error: Command rejected: An interface whose trunk encapsulation is “Auto” can not be configured to “trunk” mode?

I first got this error when I was trying to setup a Cisco 3560. To be more precise