Trunking existing Vlans to a Port Channel

[dbosch]

Hi,

My 1st post here so here's the scenario:
* You have a live port channel (Po200) on a vpc pair of cisco WS-C6509-E aggregation switches, these are going back to a cabinet switch WS-C2960G-24TC-L (CAB-10-1) 

 - Existing live Vlans 125,127 trunked.
* You need to add new Vlans (126 and 128) to the Port channel.

You prepare the following Syntax to apply to your :
 # switchport trunk allowed vlan add 126-128

Existing Po200:

#sh run int Po200
Building configuration...

Current configuration : 320 bytes
!
interface Port-channel200
description CAB-10-1
switchport
switchport trunk encapsulation dot1q
switchport trunk native vlan 2
switchport trunk allowed vlan 2,125,127,289
switchport mode trunk
end

Would the fact you adding an already existing Vlan "127" cause any issues to any connected devices already on Vlan 127?
I know this isn't best practice however I was wondering if you would see issues or possibly cause convergence issues to any connected devices.

thanks in advance.

[Richard Burts]

I have not done what you are asking so I do not speak from experience. But I would think that it should work without impact. If you are concerned about it you could certainly use this form of the command and be quite sure of no impact

switchport trunk allowed vlan add 126,128

HTH

Rick

[rasmus.elmholt]

Hi There

I have not added VLANs on the platform you mentions, but I have often added VLANs to trunk port and LAGs as well without issues. Even if the added VLAN already is allowes on the trunk.

If you only want to add VLAN 126 and 128, then just add those 2:
# switchport trunk allowed vlan add 126,128

[dbosch]

Thanks for the responses.
I am aware to add just the 2 vlans to use the suggested syntaxes as is the best practice.

I was wondering is anyone might have a definitive answer if adding as per my original request (including the existing vlan) would have any impact, What I'm trying to determine is if doing this caused an issue or whether it was something on the Hypervisor that may have caused a device connectivity to be lost.

[Richard Burts]

I am not authoritative or definitive on this issue and would be happy if someone joins the discussion who is. My opinion is that it is much more likely to be some issue in the Hypervisor than it is an issue caused by adding an allowed vlan that is already allowed.

Having said that, it may be a bit premature to say with certainty that the issue is with the Hypervisor. We do not know what kind of issue it was. And we can not rule out the possibility that your change did cause some issue. I am thinking in particular that if you make changes at one end of the port channel and there is some amount of time before you make corresponding changes at the other end of the Port Channel that this mismatch might have some impact.

The original question about whether adding an allowed vlan that is already allowed causes a problem is easier to answer. The real question about whether making changes in the port channel caused some issue is much more difficult to answer. Perhaps you have some additional information that might help us?

HTH

Rick

[dbosch]

Only addition I can post is that a Virtualization engineer resolved the issue their end by removing and re-adding the device from the Vlan. Funny thing is other devices within that Vlan were fine. So I too feel it's Hyp related.

From what it looks like by them doing that allowed convergence to happen again for the VM's affected.
So I can't say definitively whether the changes done physically on the PO caused the loss of connectivity.

Thanks

[paul driver]

Hello

---

@dbosch wrote:

Hi,

My 1st post here so here's the scenario:
*(CAB-10-1) 

 - Existing live Vlans 125,127 
* You need to add new Vlans (126 and 128) to the Port channel.

You prepare the following Syntax to apply to your :
 # switchport trunk allowed vlan add 126-128

Existing Po200:

#sh run int Po200
Building configuration...

Current configuration : 320 bytes
!
interface Port-channel200
description CAB-10-1
switchport
switchport trunk encapsulation dot1q
switchport trunk native vlan 2
switchport trunk allowed vlan 2,125,127,289
switchport mode trunk
end

Would the fact you adding an already existing Vlan "127" cause any issues to any connected devices already on Vlan 127?
I know this isn't best practice however I was wondering if you would see issues or possibly cause convergence issues to any connected devices.

thanks in advance.

No it shouldn’t  - I have done this quite few times unplanned and also times not included the ADD should I say when I have had a brain freeze and its not caused no issues in the past apart from when the range I added pruned off vlans I wanted to keep but as your range scopes all what you require then you should be fine 

[dbosch]

Thanks for the responses.
I too have done as above with no issues in the past, luckily our TACACS fails if attempting without using the "add" syntax.
I have a feeling it was Hyp related as well.
Thanks,