From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Lamparter Subject: Re: Using gretap to tunnel layer 2 traffic Date: Mon, 12 Sep 2011 14:52:50 +0200 Message-ID: <20110912125250.GA3020447@jupiter.n2.diac24.net> References: <1315589104.22676.YahooMailNeo@web45104.mail.sp1.yahoo.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: "netdev@vger.kernel.org" To: John H Return-path: Received: from spaceboyz.net ([87.106.131.203]:46380 "EHLO spaceboyz.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754037Ab1ILMwz (ORCPT ); Mon, 12 Sep 2011 08:52:55 -0400 Content-Disposition: inline In-Reply-To: <1315589104.22676.YahooMailNeo@web45104.mail.sp1.yahoo.com> Sender: netdev-owner@vger.kernel.org List-ID: On Fri, Sep 09, 2011 at 10:25:04AM -0700, John H wrote: > [...] All VLANs and physical devices have MTU= s of=20 > 1500.=C2=A0 The gretap device has a resultant MTU of 1462, thereby ca= using the=20 > bridge device to have an MTU of 1462. > [...] > The real question is, however, why is any client able to send a singl= e ICMP > packet with size 1478 bytes when one of the hops along the way only=20 > supports 1462 bytes per its MTU? Shouldn't this have been negotiated=20 > beforehand? No. An Ethernet segment needs a single, unbroken, identical MTU at all of its (packet-sending) participants. Your configuration is invalid; there is no Ethernet-builtin mechanism to negotiate MTU. You need to set all devices, hosts and possibly switches to the same MT= U value. (A switch with its default of 1500 isn't a problem as long as it does not generate packets that large.) A way to do that is by DHCP, which has a MTU option; that option is however not honoured by all clients. Alternatively, you need to change your gretap/bridge MTU to 1500, but you'll somehow need to make gretap aware of the underlying fragmentation support, from whereever it may come. Alter-alternatively, you can use a layer 3 (IP/IPv6) router. IP, unlike Ethernet, is designed to cope with varying MTUs... -David