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I read from this site http://www.javvin.com/protocolVLAN.html:

Many NICs for PCs and printers are not 802.1Q-compliant. If they receive a tagged frame, they will not understand the VLAN tag and will drop the frame. Also, the maximum legal Ethernet frame size for tagged frames was increased in 802.1Q (and its companion, 802.3ac) from 1,518 to 1,522 bytes. This could cause network interface cards and older switches to drop tagged frames as "oversized."

My question is if this type of problem pertains to antiquated NICs or if computers/printers bought within the past few years would certainly support "oversized" VLAN frames? I am planning to segregate our office network into VLANs (to optimize VoIP traffic) but I don't know if that means certain legacy devices will suddenly lose connectivity due to increased frame size.

Also, is there a way to test that a given device will support 802.1Q other than simply plugging it in and trying? (Note: I am familiar with Wireshark and could read ethernet frames off the wire if necessary.)

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I am not a VLAN expert, but my understanding is that if you set your network up correctly, only your switches and other core network infrastructure will need to be VLAN aware. As evidence, I present this quote from the source of all knowledge:

Edge switches on the corporate network are configured to insert an appropriate VLAN tag into all data frames arriving from equipment belonging to a given department. After the frames are transmitted on their respective VLANs through the corporate network, the VLAN tag is stripped before the frame leaves the VLAN-aware corporate network, and is sent to its destination, which is another computer belonging to the same department.

So the switches should handle the insertion and stripping of VLAN tags, and your edge equipment (printers, PCs, etc) should never see them.

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Yes. Pretty much the only time "edge equipment" (I like that term) should actually receive or send dot1q tagged frames is if you want that equipment to be present on more than 1 VLAN at the same time. –  Steven Monday Apr 7 '12 at 22:08
    
Thanks, I think I understand. It sounds like VLAN-capable switches pass the "oversized" VLAN-tagged frames amongst each other, but before sending a tagged frame out on a specific hardware port to an endpoint, the 802.1Q data is stripped out by the switch itself (not needed anymore at this point; the endpoint wouldn't use it) so the packet becomes 1518 bytes or less. Likewise, when the endpoint NIC sends untagged frames back to the same port on the switch, the switch inserts the VLAN data to the frame since it is pre-configured to know which port corresponds to which VLAN ID. Correct? –  Bobby Burgess Apr 8 '12 at 3:30
    
That's my understanding, yes. –  dsolimano Apr 8 '12 at 4:41

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