Cisco Support Community
Community Member

Channel mode

I have a question: Cisco always recommends to set the channel mode to desirable, according to the Output Interpreter.

I have a couple channels set to on, both sides and Problems with Aligment Errors and Runts.

My Question: Is there a chance to fix this problem, when I use the desirable mode, because than I have PaGP running, and this prevents all the problems that lead to Aligment Errors ?

Community Member

Re: Channel mode

I had the same problem once between 2 C4000: channel going down after 30 seconds whereas being set up as "on" on both sides.

The solution was to use "desirable" on one side and "auto" on the other side.

Note: if it is a trunk make sure you have same VLAN and encapsulation type on both side.

Hope it helps,


Cisco Employee

Re: Channel mode

If you have allignment errors and runt's then changing the channeling mode to on will

not resolve that problem.

The channel mode only is used for building (or not) building up the channel.

Alligments and runt's are error frames coming in. Nothing related to channeling.

Runt frame :a frame less than minimum length

Alignment Errors: Number of frames received whose size in bits is not a multiple of eight

It might be worthwile to verify the

duplex settings on both sides, and otherwise try using a different cable/port to see

if you can eliminate where the errors are coming from.

Community Member

Re: Channel mode

Thank you for your respondence. Finally there was a bad NIC responsible for the aligment errors and runts.

---- but for sure it´s always easier for some Admins to blame the Network ;-)---

But I still don´t know, why Cisco recommends desirable, when channel mode set to on works fine, except PaGP is not working.

If I only need it for the negotiation of speed/duplex, than I probably will never use it, because I never use auto - negotiation anyways.



Cisco Employee

Re: Channel mode

Cisco recommends desirable mode since the channel will negotiate its state. If there is hardware problem, software bug or incorrect behavior, causing one side of the connection to no longer channel, the far side will cease to channel as it will recognize the change in the state of the near side. This is advisable as a channel to non-channel configuration can lead to packet loss and network instability.

Also, desirable mode configurations will not cause issues if there is a channel misconfig (ie, one side off, the other on).

Community Member

Re: Channel mode

Well, that resolves my Question, thank you. But I think I can´t follow your recommendation in the next time, because there ´ll be an interrupt, when I switch from on to desirable, right.

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