What happen to locally define VLAN on a trunk connection between two switches

[Steph1963]

Hi,

In a case where VTP is not supported like on a ME-3400, I would like to know if VLANs that are only local to a switch are transported on trunk interface. If local VLAN are part of the trunk, does the downstream switch participate in the spannig tree when the VLAN is not defined within the switch.

Do we have to defined VLAN on an intermediate switch that transport VLAN to a downstream switch if the intermediate switch does not have any definition for this VLAN.

I am trying to understand what would be the best practices to assigned VLAN on a network that does not support VTP.

Should we define every VLAN that exists in the network on each switches or should we only define VLANs that are transported on the trunk across the network.

I am specially curious to find out if using a single switch as root for all VLAN make sense in a network where we have many local VLAN. Would this means that we have to define local VLAN on every switches.

VLAN pruning

My second question concerns the vtp pruning command. switchport trunk pruning vlan {add | except | none |remove}

What is the difference between add and remove.

My uderstanding of pruning is that it removes broadcast traffic from unutilized VLAN so I am not sure about the difference between add or remove.

More important, where shoud we put this command, on a downstream or upstream switch.

Thanks for your help

Stephane

[cadet alain]

Hi,

In a case where VTP is not supported like on a ME-3400, I would like to  know if VLANs that are only local to a switch are transported on trunk  interface. If local VLAN are part of the trunk, does the downstream  switch participate in the spannig tree when the VLAN is not defined  within the switch.

by default is trunk port is amember of all VLANs so broadcasts or unknow unicast for this VLAN will get forwarded this port.If you've got no VLAN defined then there is no spanning-tree instance for this VLAN.

Do we have to defined VLAN on an intermediate switch that transport VLAN  to a downstream switch if the intermediate switch does not have any  definition for this VLAN.

Yes you have to otherwise the traffic for VLAN will never get to downstream.

I am trying to understand what would be the best practices to assigned VLAN on a network that does not support VTP.

manually assign this VLAN to switches with trunk ports carrying this VLAN and switches with access ports in this VLAN.

Should we define every VLAN that exists in the network on each switches  or should we only define VLANs that are transported on the trunk across  the network.

See previous answer

I am specially curious to find out if using a single switch as root for  all VLAN make sense in a network where we have many local VLAN. Would  this means that we have to define local VLAN on every switches.

a Locl VLAN is one constrained to a access-distribution block as opposed to end-to-end VLAN which spans the entire network. spanning-tree will only have a VLAN instance if that VLAN exists on the switch

On your picture none of the switches have common VLANs so for them to communicate you'll need a L3 device

switchport trunk pruning vlan {add | except | none |remove}

add or remove a VLAN to/from the pruned VLANs on the trunk this is for VLANs pruned with VTP pruning which is configured on a VTP server.

Not to be confused with VLAN manual pruning which will remove a VLAN as member of the trunk.

VTP pruning is for unnecessary broadcasts and the other simply removes the VLAN from the trunk

Regards.

Alain.

[Steph1963]

Hi Alain,

Thanks for your reply

You are right if VLAN 1 is not defined between both switches.

In a case where VLAN 1 is defined like in the following diagram, would it be a good practice to only define VLAN 11 on SW1 and define VLAN 11 & 21 on SW1 and only allow VLAN 21 on the trunk between SW2 & SW1. I am not sure if it make more sense to use SW2  as a root for VLAN 11 & 21 or use SW1 as root for VLAN 11 and SW2 as root for VLAN 21.

Does only allowing VLAN 21 on the trunk is what you refer as manual pruning.

Does adding pruning is like adding one more VLAN on the VLAN pruning list automatically created by VTP, should this be done on the downstream swith to indicate which VLAN to prune for the upstream switch or the opposite.

Does removing pruning is like removing one VLAN from the VLAN pruning list and this should be done on the upstream switch so in such a way that the dowstream switch receive broadcast from a VLAN that the switch did not ask for.

Thanks for all your help

Stephane

[cadet alain · Accepted solution]

Hi,

You can modify the root bridge election per VLAN  if you want to load-balance traffic at L2 otherwise you can leave the default.

manual pruning is done with the switchport trunk allowed vlan command so yes that is what I meant

the add or remove should be done on the switch doing the pruning so the VTP server but there is no concept of upstream and downstream, it is just not forwarding broadcast on links where there is no need to.

Regards.

Alain.