Vlan and broadcasts

Hi guys,

Simple question but people have been giving me different (and confusing) answers on what I thought was a fairly straightforward subject.

How exactly are broadcasts handled by a switch when there are multiple VLANs?

For example, say we have switch A and switch B.

All of switch A's hosts are in Vlan 1 or 2.

All of switch B's hosts are in Vlan 1 or 3.

The two switches are trunked together.

Suppose a host in Vlan 2 sends a broadcast.

Which (if any) devices connected to switch B receive that broadcast?

Does switch A send the broadcast through the trunk link to switch B?

We're assuming there's no VTP (pruning, etc) or anything tricky going on here.

Thanks guys.

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Comments

ColbyG

A VLAN is a broadcast domain. So if a broadcast is sent it is flooded throughout the VLAN, which will go across trunk links (if the VLAN is active on the trunk) as well.

dynamik

Each VLAN is its own broadcast domain. If no devices are in VLAN 2 on Switch B, no devices will receive a VLAN 2 broadcast. Where are you getting conflicting information?

suffah

Actually I'm having an argument with two people about this.

I needed you guys to back me up.

I was pulling my hair out trying to explain that the whole point of a VLAN was that it created separate broadcast domains. They're convinced that switches flood broadcast traffic out every port despite VLANs.

ColbyG

suffah wrote: »

They're convinced that switches flood broadcast traffic out every port despite VLANs.

They're dumb.

SysAdmin4066

ColbyNA wrote: »

They're dumb.

I concur, that is one of the benefits of VLANS, segmentation of broadcast domains.

zeptobot

Now I know that no device on switch B will receive the broadcast because nothing is configured on switch B for Vlan 2. However, we have a different question below.

"Does switch A send the broadcast through the trunk link to switch B?"

I believe the answer to this question is yes. The experts around here may want to verify this though but here's my reasoning. If the trunk is configured between the two switches and there is no access list on the trunk ie: "switchport trunk allowed vlan 1,3" then the broadcast will cross over the trunk but won't touch any devices on the switch it will just die.

Can someone confirm or deny this?

APA

Without pruning or trunk security then yes... the broadcast will be passed out the trunk link and switch 2 will at that point determine it has no ports that need the broadcasts passed down.

This is why VTP pruning is an option... it allows the switch to dynamically determine what Vlans to prune and not prune from a trunk link when the downstream switch does not have any ports assigned to the relevant vlan that the broadcasts are originating within.

So in the case of VTP pruning being enabled... if that specific VLAN has been pruned from the trunk then no... the broadcasts would not be passed across the trunk link.

Otherwise manually enforcing trunk security is the other option to limit trunk traffic- 'switchport trunk allowed vlan (ID's)'......

creamy_stew

Does VTP pruning work if the switches are in transparent mode?

pureblue

No, vtp pruning won't work on the transparent mode running bridge. Cisco suggests the following if you have bridges in transparent mode:

Turn off VTP pruning in the entire network.
Turn off VTP pruning by making all VLANs on the trunk of the switch upstream to the VTP transparent switch pruning ineligible.