Etherchannel. The risks..???

Answered Question
Dec 20th, 2007

Hello,

We currently have many etherchannels in our network, most of which are legacy. These are configured in a 'mode ON' state.

We are now looking to configure all port channels in a 'mode desirable non-silent' state.

We have been advised this is the recommended mode.

I wanted to know what is the benefit of using such mode and whether it gives us any real benefit.

What are the risks for us to carry on using the Mode ON state ??

Thanks in advance for your help.

I have this problem too.
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Correct Answer by Kevin Dorrell about 8 years 11 months ago

AFAIK, in normal operation there is no risk in using the ON mode. It may even be safer that the dynamic mode.

What is the downside? If you are re-cabling and you connect a switch via a port that is in access mode, the connection will not revert to access mode. But that is not really an operation risk because it could be considered as part of the re-cabling exercise.

The benefit of using "desirable" is that you do not ever have to configure the port - it will automatically negotiate trunking mode if it can. Or not, as the case may be. But that is not really an operational risk.

Personnally, I prefer the "ON" mode, and use it on all my inter-switch links. That way I know exactly where I stand w.r.t. trunking on each link. But it does mean I have to re-configure when I replace an access connection with a trunk or vica versa.

Kevin Dorrell

Luxembourg

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Correct Answer
Kevin Dorrell Thu, 12/20/2007 - 02:01

AFAIK, in normal operation there is no risk in using the ON mode. It may even be safer that the dynamic mode.

What is the downside? If you are re-cabling and you connect a switch via a port that is in access mode, the connection will not revert to access mode. But that is not really an operation risk because it could be considered as part of the re-cabling exercise.

The benefit of using "desirable" is that you do not ever have to configure the port - it will automatically negotiate trunking mode if it can. Or not, as the case may be. But that is not really an operational risk.

Personnally, I prefer the "ON" mode, and use it on all my inter-switch links. That way I know exactly where I stand w.r.t. trunking on each link. But it does mean I have to re-configure when I replace an access connection with a trunk or vica versa.

Kevin Dorrell

Luxembourg

Jon Marshall Thu, 12/20/2007 - 02:13

Hi

As i get older my memory isn't quite as good :) but in addition to Kevin's post i seem to remember that an etherchannel that is using PagP/LACP will log a message if it goes down whereas an etherchannel that is set to on will not.

Jon

glen.grant Thu, 12/20/2007 - 04:41

Everyone has there own preferences and the cisco docs generally say to use desirable non-silent between cisco devices or you can use LACP in the active state . I'm in your camp as we make all ours desirable non-silent and have never had any issues with it . Here is a warning out of a doc pertaining to on mode. Using negotiated mode will stop this as it will not build the channel if the parameters are incorrect.

An interface in the on mode that is added to a port channel is forced to have the same characteristics as the already existing on mode interfaces in the channel.

Caution You should exercise care when setting the mode to on (manual configuration). All ports configured in the on mode are bundled in the same group and are forced to have similar characteristics. If the group is misconfigured, packet loss or STP loops might occur.

andrew.burns Thu, 12/20/2007 - 05:12

Hi,

In addition to these comments I'd like to add a couple of other points:

Plus point >> Mode ON is faster to converge (more important in the data center but always worth remembering - negotiation takes 7 seconds or thereabouts)

Minus Point >> Mode On offers no protection against various failures that would otherwise bring the channel down. (For example, an MTU mismatch or qos mismatch could result in a broken channel but with all the links still up, whereas a negotiated channel will be torn down if there are mismatches)

HTH

Andrew.

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