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broadcast traffic on trunk interfaces

Posted on 2013-12-19
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Hi - It was mentioned to me that the VLANs in our network traverse the entire campus - causing unnecessary broadcast throughout LAN.

Each edge/Access switch on each floor (each port of these) is only in the "floor VLAN' - its only the uplink trunk interfaces of these access switches that have "allowed vlan all" enabled. So the access ports dont get all broadcast. - However a Cisco engineer explained that the switches themselves are processing the broadcast traffic from other VLANs. I understand how this could be - however im not sure "what effect" this broadcast would have on the access switch- (given that only the trunk/uplink would receive the broadcast). What "pressure" then as he stated is it putting on the switch. - I understand "allowed all" should not be on - no need to mention this in answer. - thanks Im just trying to learn about broadcast on trunk interfaces - effect on "switch as whole"
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Question by:philb19
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Craig Beck earned 2000 total points
ID: 39729380
All traffic has to be processed by the switch for it to determine what to do with it.  If there are ACLs on the switch this means the traffic has to undergo further processing and that puts strain on the CPU.  This is undesirable to say the least.

Generally if traffic doesn't need to traverse a trunk on a specific VLAN you should prune that VLAN from the trunk at one end at least - usually the end closest to the origin of the traffic, or even better, at both ends of the trunk.

Following the guideline to prune unnecessary VLANs can also help to mitigate issues such as broadcast storms and STP loops.
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by:philb19
ID: 39730849
Perfect thanks
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by:philb19
ID: 39734323
Another question/point. - if broadcast traffic comes in on a trunk link. The destination MAC is unknown what happens? - it is flooded on all interfaces except the one it came in on (the trunk) - it cant be flooded on the access ports as they are not on the VLAN ? - so is it just dropped? - Am i misunderstanding something
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by:Craig Beck
ID: 39734530
You're right. The switch decides that no ports are in a VLAN which should receive the broadcast, so it just drops the broadcast.
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