Date: Thu, 3 Feb 2000 17:29:28 +0100 (MET) From: "Pedro J. Lobo" <pjlobo@euitt.upm.es> To: "Matthew N. Dodd" <winter@jurai.net> Cc: Jasper Wallace <jasper@ivision.co.uk>, Mike Nowlin <mike@argos.org>, freebsd-net@FreeBSD.ORG Subject: Re: 802.1Q VLANs Message-ID: <Pine.OSF.4.21.0002031711230.1338-100000@haddock.euitt.upm.es> In-Reply-To: <Pine.BSF.4.21.0002031052090.479-100000@sasami.jurai.net>
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On Thu, 3 Feb 2000, Matthew N. Dodd wrote: >On Thu, 3 Feb 2000, Pedro J. Lobo wrote: >> if_fxp does have to be patched. You have to tell the card not to drop >> frames with errors (oversized frames among them) or you won't receive >> packets with sizes from 1497 to 1500 bytes (remember that 802.1Q adds 4 >> bytes to each frame). Then you have to drop by hand undersized frames and >> frames with crc errors. > >If the size of our header increases then the payload we can carry >decreases. But then you won't be able to interact with other machines that send 1500-byte frames, which are perfectly legal. Keep reading... >Violating ethernet frame lengths doesn't strike me as a good idea. But you *aren't* violating ethernet frame lengths. 802.1Q frames *can* be 4 bytes longer than traditional ethernet frames. One of the strengths of 802.1Q (IMHO) is that its frames can coexist with traditional ethernet frames even in the same wire (cable, hub, etc.). The machines that doesn't recognize 802.1Q frames see them as "unknown type" frames, or as oversized frames, and simply ignore them. 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. 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. >Besides, there is little we can do about hardware that doesn't allow us to >send/receive larger packets. 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. Cheers, Pedro. -- ------------------------------------------------------------------- Pedro José Lobo Perea Tel: +34 91 336 78 19 Centro de Cálculo Fax: +34 91 331 92 29 E.U.I.T. Telecomunicación e-mail: pjlobo@euitt.upm.es Universidad Politécnica de Madrid Ctra. de Valencia, Km. 7 E-28031 Madrid - España / Spain To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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