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Cisco Etherchannel Mode

Posted on 2014-10-24
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Last Modified: 2014-10-26
When configuring Etherchannel, if I understand :
- if one switch has mode ON the other should be ON too to form a channel.
-if one switch has mode Desirable, the other switch should either be Auto or Desirable to form a channel
- if one switch has mode Active, the other is passive. will this form an Ether channel ?

***I have heard that it is best practice to stay away from mode ON. I am not sure why ?

Thanks

SW1(config-if-range)#channel-group 1 mode ?
  active     Enable LACP unconditionally
  auto       Enable PAgP only if a PAgP device is detected
  desirable  Enable PAgP unconditionally
  on         Enable Etherchannel only
  passive    Enable LACP only if a LACP device is detected
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Question by:jskfan
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by:Don Johnston
ID: 40403458
On-On = etherchannel
Desirable-Desirable = etherchannel
Desirable-Auto = etherchannel
Auto-Auto = not etherchannel

Active-Active = etherchannel
Active-Passive = etherchannel
Passive-Passive = not etherchannel

The potential problem with on-on is that you are forcing the ports into a channel even if the ports are not correctly configured.  And since spanning tree can't see the individual ports, a broadcast storm could result.  With PAgP or LACP, the ports will go error disable if a problem arises.
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Author Comment

by:jskfan
ID: 40404101
The potential problem with on-on is that you are forcing the ports into a channel even if the ports are not correctly configured
Example  ?
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Expert Comment

by:Don Johnston
ID: 40404114
All the ports that make up the group are configured as a trunk but don't have the same VLANs allowed.  Or one of them has a different native VLAN.
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Expert Comment

by:Akinsd
ID: 40404184
An easy way to overcome that problem would be to default the member ports before configuring etherchannel, and then apply configurations to the port channell.
eg
default int range gi1/1 - 4
etc
You can also use the default command on the port channell and then configure desired options eg trunk etc

Device configurations are based solely on preferences based on desired goal.
Flat generalizations to stay away from a configuration are usually based on the common configurations out there. 1 single configuration would not work for ALL situations.

Best thing is to know what your options are and what the advantanges and consequences are, and make an informed decision.

On-On gives you full control as it eliminates negotiation among other benefits. Connects faster, however, you must know what you're doing.
Negotiation guarantees consistency. You don't have to be a guru as it does most of the work for you.

As you can see below, the modes highlight the different proprietory or non-proprietory modes. PAGP (Cisco), LACP (Non-Proprietory)
http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3550/software/release/12-1_13_ea1/configuration/guide/3550scg/swethchl.html#wp1028480
Table 29-1 EtherChannel Modes  
Active
Places an interface into an active negotiating state, in which the interface starts negotiations with other interfaces by sending LACP packets. - Non-Proprietory
 
Auto
Places an interface into a passive negotiating state, in which the interface responds to PAgP packets it receives but does not start PAgP packet negotiation. This setting minimizes the transmission of PAgP packets. - Cisco
 
Desirable
Places an interface into an active negotiating state, in which the interface starts negotiations with other interfaces by sending PAgP packets. - Cisco
 
On
Forces the interface into an EtherChannel without PAgP or LACP. With the on mode, a usable EtherChannel exists only when an interface group in the on mode is connected to another interface group in the on mode. - No protocol
 
Passive
Places an interface into a passive negotiating state, in which the interface responds to LACP packets that it receives, but does not start LACP packet negotiation. This setting minimizes the transmission of LACP packets. - Non Proprietory.

You can also compare with Routing protocols.
Static - No negotiation
EIGRP Cisco Proprietory
OSPF Non Proprietory
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Author Comment

by:jskfan
ID: 40404956
Negotiation guarantees consistency. You don't have to be a guru as it does most of the work for you.
Did not know that
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Expert Comment

by:Don Johnston
ID: 40404978
I'm not sure what was meant by that, but if the physical port configurations are not identical,  PAgP or LACP will not correct that.  It will simply errdisable the ports.
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Assisted Solution

by:Akinsd
Akinsd earned 250 total points
ID: 40405110
Let me Break it down a little more.
Note my first comment about defaulting the ports. The protocol will not configure the mode for you just like OSPF will not configure your neighbors for you. Once the basic stuff for connections are configured, the protocol takes over by applying defaul parameters.
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Author Comment

by:jskfan
ID: 40405354
I conclude that LACP and PAgP are safer..?
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Accepted Solution

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Don Johnston earned 250 total points
ID: 40405366
Well, IMHO, it's definitely preferable to manually building the channel with the on command.
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Author Closing Comment

by:jskfan
ID: 40405579
Thank you Guys!
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