Adding a vlan to a trunk question

[cchughes]

In using the "switchport trunk allowed vlan 1,2,3" command, to add a vlan to the existing list of vlans allowed, will I cause spanning tree to re-converge?

I'm pretty sure it wont but I have no way to lab this before a change control that I'd like to perform remotely.

Thanks...

[adamclarkuk_2 · Accepted solution]

Hi

The switches will start to send and process BPDU's for the new vlan so as long as you haven't messed with pri / costs settings per vlan then no and even if you have, it would only affect the new vlan ( I'm assuming your using PVST+ / rapid or MST )

[Giuseppe Larosa · Accepted solution]

Hello Chris,

you can use

switchport trunk allowed vlan add newvlan

otherwise you need to rebuild the complete vlan list (including the new one)

And this should start a new instance for newvlan if using PVST+ or RPVST.

existing STP instances are not affected

With MST if the newvlan is already associated with the intended MST instance could even cause no change at all if already the MST instance is in forwarding state over the trunk.

Hope to help

Giuseppe

[tedreese]

Giuseppe, what is the best method to view the stp instances after this change? Meaning I want to check the stp instances after doing:

switchport trunk allowed vlan remove 2-4095

!

swithport trunk allowed vlan 2,3,4

!

Thanks,

[Giuseppe Larosa]

Hello Ted,

use

sh spanning-tree summary

before

and after (wait some time)

Hope to help

Giuseppe

[tedreese]

yup that's my plan. Going to get in the lab for assurance but again thanks for your assistance.

[cchughes]

Great. Thanks for the help guys...

[Tshi M]

I don't know if it is relevant here but just to add all the great inputs. If adding vlan to trunk and there is an etherchannel, make sure to add the vlan to the etherchannel first before adding it to the individual trunks. if not, you will create a loop.

rgds,

[tedreese]

I am unclear on this. My plan is to add it to the GigabitEthernet channel only and hope that it propagated the trunks. I am going to have to validate this. If not I'm doing the individual trunks as well.

[glen.grant]

You should only add them to the port channel interface itself not the indivdual ports , this will add the statements you made to all ports in the channel group at once. make sure they are exactly the same on both ends of the channel.

[Giuseppe Larosa]

Hello,

yes once the etherchannel is formed changes to the list of vlans permitted have to be done in the port-channel only.

Otherwise serious problems can occur

We had bad experiences for this reason: bridging loops with extended out of service.

So I strongly recommend once the etherchannel is formed refer to the port-channel interface for config change.

Hope to help

Giuseppe

[cchughes]

All good information. Thanks again!