Impact for the "switchport trunk allowed vlan" command use

[SRES-DGDSI@UGA]

Hello,

I was wondering if there was any impact on the performance and throughput of a switch for which interfaces are configured in trunk mode with the command "switchport trunk allowed vlan vlan-list", have you an idea ? Do you know how this "filtering" is processed : hard, soft ?

Best regards,

[JohnTylerPearce]

By default, a trunk all pas all VLANs 1-4094. So, for example, lets say you have Switch A, with VLAN 10,20, and 30, but in your entire network you have other vlans. So If anyone of those VLANs were to send a broadcast it would hit the trunk port on Switch A, since that port is technically a member of all VLANs. So, you can have performance issues if something like this occurs. It also depends on the enviornment. Also if you do 'switchport trunk allowed vlan vlan-list' make sure to understand that ONLY those vlans will be able to pass unless yo uadd on to that.

Word of Caution: If you need to add a vlan to a trunk do not do the following

'switchport trunk allowed vlan 10' If you do this it will only allow vlan 10 and NOTHING else.

Do this 'switchport trunk allowed vlan add 10' That will add 10 along with the others.

Hope that helped some.

[Alessio Andreoli]

Hi NetAdm,

no impact will be noticed once you will issue this command. the only thing you need to pay attention to is the vlan-list. If in that list you do not specify all the vlans you want to go through the trunk, you will lose some traffic. Also, when you specify this list make sre that the native vlan is included in this list (the default on most of the devices is VLAN 1). for what concerns the performance you just need to think that

switchport trunk allowed vlan vlan-list

is just telling to the trank which vlan tags are allowed on that pipe. So the impact should not even be a concern for you unless very particular configurations or settings.

Hope this helps

Alessio

[Giuseppe Larosa]

Hello,

there is no performance degradation in the use of the switchport trunk allowed vlan vlan-list.

I have seen it used in server farms. You are declaring of what Vlans the trunk is member of, and this action is mapped in the switch hardware

Actually, the use of manually configured lists of Vlans allowed on trunk links is a key point for achieving scalability in STP when dealing with low end switches that have a limited number of STP instances running on the box.

Simply Cisco default for a trunk port is to allow all vlans, other vendors like Juniper have a different default of "none" vlan allowed on the trunk by default.

This is key difference between

switchport trunk allowed vlan vlan-list

and VTP pruning is this: VTP pruning does block flooding of broadcast, unknown unicast, multicast for Vlans without users downstream, but it does not limit the execution of STP on downstream nodes for those Vlans without users.

So the use of manually configured trunks with switchport trunk allowed vlan command is recommended to save resources on downstream switches both in the forwarding plane and in the control plane.

Edit:

John has given evidence to the biggest operational impact of this command:  after initial configuration of the trunk, later changes to the list of permitted vlans has to be performed using the switchport trunk allowed vlan add.

Hope to help

Giuseppe