Adding a vlan to a trunk question

Answered Question
Feb 10th, 2009

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...

Correct Answer by Giuseppe Larosa about 8 years 1 week ago

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

Correct Answer by adamclarkuk_2 about 8 years 1 week ago

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 )

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Correct Answer
adamclarkuk_2 Tue, 02/10/2009 - 12:11

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 )

Correct Answer
Giuseppe Larosa Tue, 02/10/2009 - 12:19

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 Thu, 02/12/2009 - 11:09

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 Thu, 02/12/2009 - 11:48

Hello Ted,

use

sh spanning-tree summary

before

and after (wait some time)

Hope to help

Giuseppe

tedreese Mon, 02/16/2009 - 11:07

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

Tshi M Mon, 02/16/2009 - 13:08

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 Mon, 02/16/2009 - 13:13

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 Mon, 02/16/2009 - 14:03

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 Mon, 02/16/2009 - 14:07

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

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