Date: Tue, 04 Jul 2000 16:25:12 -0700 From: "Kevin Oberman" <oberman@es.net> To: "Louis A. Mamakos" <louie@TransSys.COM> Cc: David Gilbert <dgilbert@velocet.ca>, Joerg Micheel <joerg@cs.waikato.ac.nz>, freebsd-stable@FreeBSD.ORG Subject: Re: Ethernet MTUs > 1500? Message-ID: <200007042325.e64NPCn20451@ptavv.es.net> In-Reply-To: Your message of "Tue, 04 Jul 2000 01:08:36 EDT." <200007040508.BAA59763@whizzo.transsys.com>
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> From: "Louis A. Mamakos" <louie@TransSys.COM> > Date: Tue, 04 Jul 2000 01:08:36 -0400 > Sender: owner-freebsd-stable@FreeBSD.ORG > > > > >>>>> "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. Louis has it exactly right. 802.3 was modified a couple of years ago to allow for a maximum frame size of 1522 octets, up from the original 1518. This was to allow the VLAN information to fit in the frame. Since the terminating switch/router should be removing this information, the end system (and MTU) should be unaffected, but many nets seem to not fully support the longer frames in their infrastructure and probably lots of Internet routers don't either. This is often a hardware issue, so the only fixes available are to upgrade all hardware to support the added octets or to reduce the MTU so that the frame size does not exceed 1518. In an enterprise, the upgrade is feasible, but in the Internet backbone, if you are not extremely lucky to have all VLAN supporting equipment along the path, the packet is fragmented or dropped. And there is a LOT of equipment out there that does not support VLANs. R. Kevin Oberman, Network Engineer Energy Sciences Network (ESnet) Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab) E-mail: oberman@es.net Phone: +1 510 486-8634 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
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