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on a trunk, switch A having, say vlan 10 allowed and the other switch B does not. Switch B doesn't even know about that vlan. How does switch A behave in forwarding broadcasts in vlan 10? Will frames be forwarded to switch B and utilize bandwidth of the link?

Another example with a server interface: The server has a router-on-the-stick like config with vlan subinterfaces and simply all vlans are allowed on the trunk configured at the server facing switch interface. Will broadcasts from all vlans be forwarded out of the switchinterface to the server (and he'll need to discard)?

Hopefully this question doesn't dublicate any existing and easy to find one ;)

One thought about that I had, was the vlan active on a trunk or not thing. Maybe behavior could differ in a setup as described in the second example.

Thanks for your answers!

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When VLAN 10 is tagged on egress from switch A but unknown in switch B the frames will be sent on the link and dropped by switch B.

When VLAN 10 is untagged on the egress port all frames enter the untagged VLAN on switch B.

The same with your second question: the switch sends traffic for all allowed VLANs on the link, whatever the receiver chooses to do with them.

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  • That would explain some things. Ingress discard counters rising at distribution switches when all vlans are allowed 'upwards' fe.
    – René
    Commented Sep 19, 2017 at 18:13

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