Date: Thu, 3 Feb 2000 12:40:47 -0500 (EST) From: "Matthew N. Dodd" <winter@jurai.net> To: "Pedro J. Lobo" <pjlobo@euitt.upm.es> Cc: freebsd-net@FreeBSD.ORG Subject: Re: 802.1Q VLANs Message-ID: <Pine.BSF.4.21.0002031236160.479-100000@sasami.jurai.net> In-Reply-To: <Pine.OSF.4.21.0002031711230.1338-100000@haddock.euitt.upm.es>
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On Thu, 3 Feb 2000, Pedro J. Lobo wrote: > You can even think of another scenario (which is what I have here): > you can configure one port in a switch as a "tagged" port (i.e. it > will expect and transmit 802.1Q frames) and another one as a > "untagged" port (i.e. a traditional ethernet port, an both of them can > belong to the same VLAN. When a machine plugged to the untagged port > sends a 1500-byte frame, it will appear at the tagged port 4 bytes > longer. You can check it with a sniffer. And it is perfectly legal. Yea, that makes a bit more sense. > The 802.1Q standard does allow frames with those 4 extra bytes, and > the machines that "speak" 802.1Q must be able to recognize those long > frames as valid. Humm... Ideally the VLAN driver should be able to set the MTU for its parent and deal with being able to set the MTU > 1500. This prevents us from making VLAN specific hacks to the other drivers and relegates the problem to one of supporting 'large frames'. > You are completely right on this. Not all cards will be able to > support 802.1Q. The Etherexpress Pro/100 (fxp driver) certainly is. I > don't know about other cards, but I suspect that many of them will > offer you the posibility of send and receive extra-large frames. Well, I see no reason to restrict people to cards that don't support large frames; if they really need to use VLANs they can adjust the MTU down. In the real world MTU discovery will insure that they aren't too big of a problem. -- | Matthew N. Dodd | '78 Datsun 280Z | '75 Volvo 164E | FreeBSD/NetBSD | | winter@jurai.net | 2 x '84 Volvo 245DL | ix86,sparc,pmax | | http://www.jurai.net/~winter | This Space For Rent | ISO8802.5 4ever | To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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