Etherchannel for an 3550 Catalyst and maybe others

Etherchannel is for fault tollerance and you can use it to increase the bandwidth. You can bound up to to eight interface to an eitherchannel. All interfaces must be the same speed and all have to be configured as layer 2 or layer 3 interfaces. If one interface fails, the traffic is automatically redirected to the other interfaces.

Etherchannel can be configured in one of these modes:

  • Port Aggregation Protocol (PAgP)
  • Link Aggregation Control Protocol (LACP)
  • On mode

You can setup an etherchannel in two different ways. You can set it up as layer 2 or a layer 3 interface.

There are four etherchannel modes:

  • active
  • auto
  • desirable
  • passive

Only in auto and desireable switches are exchanging PAgP packets. Only in active and passive switches are exchanging packets in LACP mode.

The following cominations are allowed for forming an etherchannel:


auto – desirable

desirable – auto

desirable – desirable


active – active

active – passive

There is also on – mode, witch is for not negotiating the etherchannel between the devices. Could be usefull if one device does not support PAgP or LACP.

Here is an configuration example, where Switch 1 is trying actively to convert this link in an etherchannel, while Switch 2 will respond to these requests. The Link is also configured as a trunk link with dot1q:


Ports fa0/13 – 15 to be configured in the etherchannel.

sw3(config-if)#int range fa0/13 – 15
sw3(config-if-range)#switchport mode trunk

sw3(config-if-range)#channel-group 1 mode desirable
Creating a port-channel interface Port-channel 1
02:19:53: %LINK-3-UPDOWN: Interface Port-channel1, changed state to up
02:19:54: %LINEPROTO-5-UPDOWN: Line protocol on Interface Port-channel1, changed state to up

SW2(config)#int range fa0/13 – 15
SW2(config-if-range)#switchport trunk encapsulation dot1q

SW2(config-if-range)#switchport mode trunk
SW2(config-if-range)#channel-group 1 mode auto
Creating a port-channel interface Port-channel 1
01:39:20: %LINK-3-UPDOWN: Interface Port-channel1, changed state to up
01:39:21: %LINEPROTO-5-UPDOWN: Line protocol on Interface Port-channel1, changed state to up

Here are some commands for verifying etherchannel functions. CDP is working over etherchannel, so if you see your other switch, then etherchannel will work.
To verify the etherchannel use the commands:

show etherchannel

show pagp

show lacp

show cdp


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