LAN-VLAN theory question

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Feb 22nd, 2009
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Here is my situation...

As many of you know, by default, vlan trunks carry ALL vlan traffic to each switch.

However, if i just had the trunks carry ONLY the VLAN traffic that it needs to each switch, would i see a big difference in speed on my network?

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mattkaya56 Sun, 02/22/2009 - 19:15
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I think you meant trunks between switches with only the default vlan.

Any vlan processing delays things in an asic - tag inspection etc. So, if you have a flat network with no vlans trunks will not speed things faster than just bridging.

Experts please confirm.

mattkaya56 Sun, 02/22/2009 - 19:17
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If your switches support stacking it will speed things up.

To the experts again

accesshollywood2 Sun, 02/22/2009 - 19:19
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actually, i have about 20 VLANS on my network and about 15 switches spread throughout. However we only use about 10 VLANS, the remaining VLANS are going to our tennants in the building. Does that help?

mattkaya56 Sun, 02/22/2009 - 21:41
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1) Hub and spoke to minimize the number of hops. The HUB switch will connect to L3 switch/router and to the internet.

2) Enable trunk on each spoke. make sure you prune(VLAN).

I assumed computers on each VLAN do not have to talk to each other - each VLAN represents a tennant. If vlans need to talk create static routes.

Experts please confirm.

davebarus Sun, 02/22/2009 - 21:41
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If you are using VTP to propagate your VLANs to other switches then I'd suggest using the VTP Pruning feature. It will prevent unnecessary VLAN traffic (broadcasts/multicasts) from being sent to switches that don't require it. If you are not using VTP you can specifically allow/disallow VLANs manually on your trunks by using the following command on the trunk interface:

router(config-if)#switchport trunk allowed vlan [vlan number]


router(config-if)#switchport trunk pruning vlan [vlan number] ...

As far as noticing a BIG difference in speed, usually not. Although, it depends on your network. Most unnecessary VLAN traffic that will go across your trunks will probably be broadcasts/multicasts.

Giuseppe Larosa Mon, 02/23/2009 - 03:41
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Hello Dustin,

if you mean that you have configured the trunk links with

switchport trunk allowed vlan vlan-list

this is good practice that I would recommend:

STP instances are kept to a minimum on access layer switches (that have limits both on max STP instances, and max Vlans supported).

This provides scalability

Efficiency is provided by avoiding unnecessary broadcast/multicast and unknown unicast traffic to be propagated everywhere

This is a performance gain and can be achieved also with VTP pruning but VTP pruning doesn't minimize the number of STP instances running.

So compared to carrying all vlans on trunks without even VTP pruning you save bandwidth resources on links.

Inside each vlan the CAM table dictates where the traffic has to be sent:

regular traffic with a known unicast destination is not propagated anywhere.

What can be pruned is the unnecessary traffic.

A big difference in speed is not experienced (unless the amount of broadcast/multicast traffic is high) but the network has more space to grow in the future (more scalability and more efficiency)

Hope to help


Joseph W. Doherty Mon, 02/23/2009 - 04:30
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"However, if i just had the trunks carry ONLY the VLAN traffic that it needs to each switch, would i see a big difference in speed on my network?"

Perhaps or perhaps not; it depends. It depends on how much needless traffic is being sent across the trunk, the impact of the needless traffic to the processing of the downstream switches and the processing features of the switches.

On a "typical" LAN, it's unlikely you would see much of a difference. This assuming most traffic is unicast and the switch doesn't need to forward the traffic across the trunk.

However, even beyond other posters mention of routine broadcast, multicast, VTP and STP issues, there's also security to consider. Since you mention tenants in your second post, "routine" traffic might not always be the case. Tenants might be more likely to create an abnormal situation which properly pruned vlans might help mitigate. In other words, it's generally a good idea to contrain vlans to only where needed, but perhaps even more so in your environment even if you don't see any immediate speed improvement.


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