From owner-freebsd-net@freebsd.org Mon Jul 13 08:51:46 2015 Return-Path: Delivered-To: freebsd-net@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 963873349 for ; Mon, 13 Jul 2015 08:51:46 +0000 (UTC) (envelope-from steve.read@stormshield.eu) Received: from work.netasq.com (gwlille.netasq.com [91.212.116.1]) by mx1.freebsd.org (Postfix) with ESMTP id 5EE3C1901 for ; Mon, 13 Jul 2015 08:51:44 +0000 (UTC) (envelope-from steve.read@stormshield.eu) Received: from work.netasq.com (localhost.localdomain [127.0.0.1]) by work.netasq.com (Postfix) with ESMTP id A5DFF2706088 for ; Mon, 13 Jul 2015 10:51:42 +0200 (CEST) Received: from localhost (localhost.localdomain [127.0.0.1]) by work.netasq.com (Postfix) with ESMTP id 87B4D2706079 for ; Mon, 13 Jul 2015 10:51:42 +0200 (CEST) Received: from work.netasq.com ([127.0.0.1]) by localhost (work.netasq.com [127.0.0.1]) (amavisd-new, port 10026) with ESMTP id PdvA6zuxcBuR for ; Mon, 13 Jul 2015 10:51:42 +0200 (CEST) Received: from stever.netasq.com (unknown [10.2.39.1]) by work.netasq.com (Postfix) with ESMTPA id 628D52705BD5 for ; Mon, 13 Jul 2015 10:51:42 +0200 (CEST) Message-ID: <55A37C1E.90804@stormshield.eu> Date: Mon, 13 Jul 2015 10:51:42 +0200 From: Steve Read User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0 MIME-Version: 1.0 To: freebsd-net@freebsd.org Subject: Re: lagg of em0/em1 + VLAN = lower MTU? References: <7CFE75F7566F5789DAD9FBB2@[10.12.30.106]> In-Reply-To: <7CFE75F7566F5789DAD9FBB2@[10.12.30.106]> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-BeenThere: freebsd-net@freebsd.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: Networking and TCP/IP with FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 13 Jul 2015 08:51:46 -0000 On 10.07.2015 15:04, Karl Pielorz wrote: > > The MTU on lagg0.10 has shrunk by 4 (size of VLAN tag). Is there a way > of avoiding that? No. Think about what it means. The MTU on the lagg0 interface is the largest packet it can send for you or for its VLAN interfaces. The MTU on the lagg0.10 (VLAN) interface is the largest packet *it* can send for you. The VLAN tag is added to the packet that you give to lagg0.10, and so the MTU of lagg0.10 must be smaller than the MTU of lagg0. Crudely speaking, the sequence is: * The TCP/IP layers create a packet of N bytes (starting with an IP header), and give them to the "ethernet" layer (lagg0.10) which adds 14 bytes of MAC/ethertype, for a total of N+14. This must fit into the the MTU of lagg0.10. * lagg0.10 then adds four more bytes between the MAC/ethertype and the IP packet, for a total of N+14+4. This must fit into the MTU of lagg0. * lagg0 then sends the packet. Note that the packet given to lagg0 is four bytes bigger than the packet given to lagg0.10, and this is always true. If you make the MTU of lagg0.10 equal to the MTU of lagg0, then lagg0.10 will end up generating packets that are oversize for lagg0, and you don't want that. -- Steve Read