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

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?

1. How it handles frames with 0x8100 in the EtherType field¹
2. 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}

Solution 1:

I have actually seen this on a cheapo-switch. Someone had connected a switch between a trunk port which had a couple vlans. The frames were forwarded with the vlan tagging intact. The other ports on that switch where able to use the un-tagged vlan.

A switch only needs the source/destination mac to decide which ports to forward the frames to, so this isn't too surprising, a tagged frame still has the source and destination macs, in the same location in the frame header.

Keep in mind that Ethernet actually supports many different frame-types on the same wire. It was designed to be pretty flexible about what it can do.

Solution 2:

Usually overly large ethernet frames can be and are discarded. In the presence of things like jumbo sized frames large ethernet frames are hard to define, so it really depends - but discarding will be probably the most frequent behavior encountered.

edit: To elaborate: Standard IEEE 802.3 Ethernet frame size is 1518 bytes, 802.3Q adds 4 bytes to the frame so has a total MTU of 1522 bytes which might be too large for some switches.