Date: Tue, 04 Jul 2000 01:08:36 -0400 From: "Louis A. Mamakos" <louie@TransSys.COM> To: David Gilbert <dgilbert@velocet.ca> Cc: Joerg Micheel <joerg@cs.waikato.ac.nz>, freebsd-stable@FreeBSD.ORG Subject: Re: Ethernet MTUs > 1500? Message-ID: <200007040508.BAA59763@whizzo.transsys.com> In-Reply-To: Your message of "Tue, 04 Jul 2000 00:37:44 EDT." <14689.27160.907313.347624@trooper.velocet.net> References: <14689.22689.894466.908666@trooper.velocet.net> <20000704153914.C60136@cs.waikato.ac.nz> <14689.23903.87264.511506@trooper.velocet.net> <200007040353.XAA03131@whizzo.transsys.com> <14689.27160.907313.347624@trooper.velocet.net>
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> >>>>> "Louis" == Louis A Mamakos <louie@TransSys.COM> writes: > > Louis> There's some confusion here, because the MTU is typically > Louis> associated with a protocol stack like IP and refers to the > Louis> largest sized (IP in this case) packet that can be sent on the > Louis> network interface. In the case of Ethernet interfaces that > Louis> support and use VLAN tags, the MTU is still 1500 bytes, > Louis> regardless of the fact that the frame size is a few bytes > Louis> longer to accomodate the VLAN tag information. > > Louis> So, this has no effect on the Ethernet type field (or 802.3 > Louis> length fields) since the higher level protocol packet size is > Louis> unchanged. > > Why, then, are the vlan MTUs hardwired at 1496? You got me. Perhaps the code is busted. Perhaps the code is trying to accomodate ethernet NICs that cannot send or receive larger than "normal" ethernet frames. (e.g., those with VLAN tags or priority labels). I just know that on networks that I've built and used with Ethernet switches which use VLAN tagging, I've had no problems transporting 1500 byte MTU IP packets inside of ethernet frames. If the NIC card in the FreeBSD host can't send the larger frames to accomodate the VLAN tags, then it's broken. louie To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
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