Vlan pruning on etherchannel

Unanswered Question
Nov 16th, 2009

Dear friends,

When doing vlan pruning for an etherchannel between two switches, if i allow only few vlans through the etherchannel on one end and let the other end permit all vlans, will it bring down the etherchannel?

I just wanted to know that is there a condition that both ends of an etherchannel must permit the same vlans to remain up, else they will go down.

Sorry if this was a stupid question, but since i am going to apply this on a production network, i just wanted to be sure about it.

Thanks a lot

Gautam

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Mohamed Sobair Mon, 11/16/2009 - 02:32

Hi,

Yes, Actually Etherchannel wont form.

To Successfully bring up an ehterchannel, The member ports on both endes has to have identical configuration.

HTH,

Mohamed

gautamzone Mon, 11/16/2009 - 02:41

Thanks a lot.

Do you know if there is a way to do it tactfully?

I am thinking that if i telnet to both ends at the same time, can i make the changes immediately and still have the port-channel up?

I know that for a trunk link, i can have all vlans allowed and on the other end, and only a few vlans allowed on the other end and still have the trunk up. But i dont know for the etherchannel.

Thanks and Regards

Gautam

Mohamed Sobair Mon, 11/16/2009 - 02:49

Hi,

unfortunately , I dont know of a way to do it tactfully.

but try login to both switches at the same time and apply the changes. I think its the best & only option for keeping the connection active. But I am not sure.

HTH

Peter Paluch Mon, 11/16/2009 - 03:12

Gautam,

It is not a stupid question at all. I had to lab it as I was not completely sure.

Mohamed has indicated that the Etherchannel will go down. I cannot confirm that. While there are some configuration characteristics that must be identical on all ports in an Etherchannel on both switches, the list of permitted VLANs is not among them.

Nothing wrong will happen if you permit a few VLANs on one end of an Etherchannel (using the "switchport trunk allowed vlan") and leave all VLANs permitted on the other end. I have also tested various combination of VTP VLAN pruning and it behaved in the same way - the Etherchannel will stay up.

Best regards,

Peter

glen.grant Mon, 11/16/2009 - 06:44

IT probably depends if you are doing dynamic trunking and etherchannel formation or if you are forcing them both on . If they are forced on it probably doesn't matter that both ends are the same whereas the negotiated protocols check a lot of that stuff and won't form if they don't match .

Peter Paluch Mon, 11/16/2009 - 06:58

Hello Glen,

You are correct. I have been doing experiments with LACP in my last post but as I indicated, the LACP allowed forming the Etherchannel even if the allowed VLAN list or list of pruning-eligible VLANs was different on both ends.

Best regards,

Peter

gautamzone Mon, 11/16/2009 - 11:00

Dear Peter and Glen,

Thanks a lot for the valuable information.

Even i got a chance to lab it out later during the day between 6500 and 4948.

Channel mode was on and i was saying sw nonegotiate on both ends.

I tried changing the native vlan, allowed vlans on one side only but the port channel stayed up still.

Thanks a lot

Gautam

Peter Paluch Mon, 11/16/2009 - 12:00

Gautam,

You are welcome.

You may do experiments with whatever VLAN stuff on your trunks except the native VLAN. Do not never ever create a native VLAN mismatch on a trunk. By this, you will eventually cause one VLAN leaking into another and you risk the trunk being temporarily disabled because of native VLAN mismatch detection in Cisco's Spanning Tree Protocol implementation. Have a look at this document:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00801d11a0.shtml

Best regards,

Peter

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