What happens when a consumer switch receives a VLAN-tagged Ethernet frame?

Posted by netvope on Server Fault See other posts from Server Fault or by netvope
Published on 2011-11-22T20:45:20Z Indexed on 2011/11/24 1:55 UTC
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Filed under:

switch

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vlan

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trunk

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802.1q

Suppose you connect a trunk port from a VLAN capable network switch to a (VLAN incapable) consumer-grade network switch via a direct cable. Now the former switch send the later switch a 802.1Q-tagged Ethernet frame. What should the later switch do? Drop the frame? Forward the frame? Undefined behavior?

If the behavior is undefined, what is most probable?

Edit: Thank you for your answers. To summarize, the behavior of the consumer switch depends on:

How it handles frames with 0x8100 in the EtherType field¹
How it handles jumbo frames, or frames with payload larger than 1500 bytes

Wikipedia has a nice diagram comparing an untagged and a tagged Ethernet frame:

There are reports that some consumer-grade switches pass VLAN-tagged frames just fine.

_{¹ or more precisely, where an EtherType field is expected for non-tagged frames}

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