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Beginner

VLAN traffic question

I have a network that has layer 2 links across all the switches the only layer 3 routing is done at the distro switch.  Now I use VTP to redistribute all the VLANs across all the switches.  Some of the switches do not use all the VLANs.  I have someone telling me that having unused VLANs on a switch will cause unnecessary traffic due to broadcasting on said VLAN.  When I do a show mac address-table vlan XXX on a switch that isn't using that VLAN it has all the MAC addresses from other devices in that VLAN from other switches.  They are saying this is the problem.  Does having an unused VLAN on a switch cause traffic to it?  And if so is there some document to show how much traffic if any? 

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Cisco Employee

Hi!

Hi!

It will, yes. Broadcasting, Unknown Unicasts, ARPs, etc will be seen in the VLANs of those switches. How much traffic will be seen depend on many factors regarding your network design, also how much it would affect your switches depends on the switch capacity itself.

VTP Pruning is the tools used to avoid that kind of traffic, since it denies unnecessary traffic because switches communicate they have no access ports belonging to x vlan so it is not necessary to send traffic to them. Please refer to the following link for detailed information about VTP Pruning:

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4500/12-2/25ew/configuration/guide/conf/vtp.html

Basically, use  switch(config)#vtp pruning

Hop it helps, Best regards!

JC 

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VIP Expert

duplicate post

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Posting

As Carlos has already noted, trunks will forward any VLAN frames that is doesn't have a unicast MAC in its MAC table.

As he also noted, VTP provides a pruning feature that will block VLAN traffic, down trunks, when there are no ports in that VLAN downstream.

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