Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
New Member

Quick Vlan Trunk Query


I recently made change on a switch stack.

It was simple case of adding vlans to a switchport trunk.

The command used was:-

switchport trunk allowed vlan add x

When the vlans were added, they removed the old vlans and caused a major outage.

I know I used the correct command because I used the up arrow to add the other Vlans.

Has anyone witnessed this before?

Kind Regards

Christian De'Ath


Quick Vlan Trunk Query

  It shouldn't happen ,  if you forgot the "add" command then yes it would make a difference.  Have also seen if you use negotiated trunks "switchport mode dynamic desirable"   that you should get both sides of the trunks done in a short period of time or the trunks will go down due to being mismatched .  I usually just open up 2 windows in this case and add the commands  quickly on each screen .  I wonder if the trunk had all vlans allowed  and when you put your command in it just put the one you added into the trunk ???

New Member

Re: Quick Vlan Trunk Query

Yes I thought that but I added each vlan seperately and pressed the up arrow to add each vlan to make sure each one had taken. There were a few vlans set up to be accepted originally but as I say, adding those other caused it to spit its dummy out.

oh well, I wonder if it was just one of things.

typically it happened within a coupe of weeks of starting a new job


Quick Vlan Trunk Query

If there was previously no restriction on the VLANS i.e

no switchport trunk allowed vlan

and you entered

switchport trunk allowed vlan add N

It would have the effect you describe. Plus you would be a hero for discovering there poor security (_)


Posted by WebUser Stuart Gall

Quick Vlan Trunk Query

I have run into that as well so I do not use the "add" method, now do it this way...

Lets say I have this configured

switchport trunk allowed vlan 3,30-34,104,601,603-606,630,633,643,703,730,733

and I need to add VLAN's  20,500,501

I create this in note pad

switchport trunk allowed vlan 3,20,30-34,104,500,501601,603-606,630,633,643,703,730,733

and then int the interface config mode I just cut and paste the new line in, I have not had an issue yet.



Quick Vlan Trunk Query

  Can't say I have ever had an issue just using switchport trunk allowed vlan add XX .  I have even used it in a interface range command before without issue.

CreatePlease to create content