From owner-freebsd-net@freebsd.org Mon Jul 13 12:52:33 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 3F48D999EC5 for ; Mon, 13 Jul 2015 12:52:33 +0000 (UTC) (envelope-from kpielorz_lst@tdx.co.uk) Received: from mail.tdx.com (mail.tdx.com [62.13.128.18]) by mx1.freebsd.org (Postfix) with ESMTP id 0D78B1A4A; Mon, 13 Jul 2015 12:52:32 +0000 (UTC) (envelope-from kpielorz_lst@tdx.co.uk) Received: from [10.12.30.106] (vpn01-01.tdx.co.uk [62.13.130.213]) (authenticated bits=0) by mail.tdx.com (8.14.3/8.14.3/) with ESMTP id t6DCqTLs032113 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Mon, 13 Jul 2015 13:52:30 +0100 (BST) Date: Mon, 13 Jul 2015 13:52:30 +0100 From: Karl Pielorz To: Gary Palmer cc: freebsd-net@freebsd.org, Steve Read Subject: Re: lagg of em0/em1 + VLAN = lower MTU? Message-ID: <558FF7CA779176D3F35B6D99@[10.12.30.106]> In-Reply-To: <20150713120651.GA68542@in-addr.com> References: <7CFE75F7566F5789DAD9FBB2@[10.12.30.106]> <55A37C1E.90804@stormshield.eu> <20150713120651.GA68542@in-addr.com> X-Mailer: Mulberry/4.0.8 (Win32) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Content-Disposition: inline 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 12:52:33 -0000 --On 13 July 2015 13:06 +0100 Gary Palmer wrote: > Have you read the HARDWARE section of vlan(4)? Kind of cryptic answer ;) - But I just read vlan(4). So it looks like my understanding of vlans (i.e. 'long frames' as it calls them) was right? On supported kit - creating a sub interface on a VLAN on will not reduce the MTU of that sub interface, e.g. to 1496. em is listed as Hardware Tagging supported, but is not listed under the 'natively support long frames for vlan' in that man page. But I can set the MTU on em to anything I want, e.g. 1504, 1600, anything up to 9k on these cards - that's a little confusing. This still doesn't really solve the question of why the MTU remains at 1500 (as I think it should) when adding a VLAN to a standalone em interface, but it 'shrinks' to 1496 when the parent interface is a LAGG of em's. Even if the actual LAGG (and it's members) are forced to an MTU 1504 first before the VLAN interface is created (which is useless in this environment anyway), the created VLAN interface comes up with an MTU of only 1496 (i.e. not '1504 -4' if it were doing that maths). End of the man page says, "The vlan driver automatically recognizes devices that natively support long frames for vlan use and calculates the appropriate frame MTU based on the capabilities of the parent interface." I would ask - does that include if the parent interface (lagg) has another parent (i.e. em0/em1)? - It would seem to in at least some cases (i.e. the other people who seem to have this working). -Karl