Date: Mon, 03 Jul 2000 23:53:20 -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: <200007040353.XAA03131@whizzo.transsys.com> In-Reply-To: Your message of "Mon, 03 Jul 2000 23:43:27 EDT." <14689.23903.87264.511506@trooper.velocet.net> References: <14689.22689.894466.908666@trooper.velocet.net> <20000704153914.C60136@cs.waikato.ac.nz> <14689.23903.87264.511506@trooper.velocet.net>
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> >>>>> "Joerg" == Joerg Micheel <joerg@cs.waikato.ac.nz> writes: > > Joerg> On Mon, Jul 03, 2000 at 11:23:13PM -0400, David Gilbert wrote: > >> But a number of devices seem to allow for MTUs > 1500 on 100Mb > >> ethernet... and several people have told me that the standard > >> allows for packets bigger than 1500 bytes. > > Joerg> The limit is 1536. That is hex 0x600, a value of importance if > Joerg> using 802.x networks. A value below 0x600 indicates the length > Joerg> of an additional header in the EtherType field. If this value > Joerg> is above 0x600 it indicates a certain protocol (such as 0x800 > Joerg> for IP, 0x806 for IP ARP). > > Joerg> I would not twiddle with the settings, the extra gain is very > Joerg> low and you'll face all sorts of compatibility issues with > Joerg> devices not supporting it. > > Well... all I'm looking for is the ability to set 1504 on the etnernet > device so that 1500 byte packets will pass on the VLANs. But at least > the "dc" driver ignores MTUs larger than 1500. There's some confusion here, because the MTU is typically associated with a protocol stack like IP and refers to the largest sized (IP in this case) packet that can be sent on the network interface. In the case of Ethernet interfaces that support and use VLAN tags, the MTU is still 1500 bytes, regardless of the fact that the frame size is a few bytes longer to accomodate the VLAN tag information. So, this has no effect on the Ethernet type field (or 802.3 length fields) since the higher level protocol packet size is unchanged. louie To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
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