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netvope
Asked: 2011-11-23 12:45:20 +0800 CST

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

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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:

  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:

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

vlan 802.1 switch trunk

3 Answers

  1. Best Answer

    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.

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  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.

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  3. The consumer-class switch will attempt to forward the frame -destination MAC address is all it cares about. If the destination MAC address is not in its CAM table, it will flood the frame out of all its ports, except the one the packet was received from.

    A switch that uses Cut Through forwarding method will definitely forward the frame, since it begins forwarding as soon as the destination MAC address is read -even if the total size of the frame is greater that the MTU -since it can't calculate the size of the frame with this forwarding method.

    A switch based on Store And Forward technique will probably (as long as the frame size is <= MTU) do the same, as long as the FCS is OKAY.

    If the 802.1Q-incapable switch interconnects end devices, the devices will receive the frame and discard it, since they don't "know" how to process 802.1Q (type 0x8100) frames.

    I speculate if the consumer-class switch interconnects 802.1Q capable switches (the horror!), the frames will be forwarded and processed by the 802.1Q -as long, of course, as they're received on trunk ports.

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