From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rick Jones Subject: Re: [PATCH] vlan: propogate MTU changes Date: Mon, 06 Oct 2008 15:50:50 -0700 Message-ID: <48EA964A.6060503@hp.com> References: <20081006173024.2741cc01@speedy> <48EA369F.3090306@trash.net> <20081006195446.1dc5a372@speedy> <48EA9223.8090700@trash.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Cc: Stephen Hemminger , "David S. Miller" , netdev@vger.kernel.org To: Patrick McHardy Return-path: Received: from g5t0007.atlanta.hp.com ([15.192.0.44]:16195 "EHLO g5t0007.atlanta.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752649AbYJFWuy (ORCPT ); Mon, 6 Oct 2008 18:50:54 -0400 In-Reply-To: <48EA9223.8090700@trash.net> Sender: netdev-owner@vger.kernel.org List-ID: Patrick McHardy wrote: > Stephen Hemminger wrote: > >> On Mon, 06 Oct 2008 18:02:39 +0200 >> Patrick McHardy wrote: >> >>> - the stack in fact doesn't require us to reduce the MTU of a VLAN >>> device as long as its within the physically possible MTU. >> >> >> I think it should call change_mtu so userspace gets notified about both >> changes. > > > Agreed. But the question when to do automatic adjustments remains. A matter of interpretation of the principle of least surprise right? Which is less surprising - that a VLAN's MTU drops to match that of the physical interface or that some traffic on the VLAN stops when the physical interface's MTU drops? If physical interface MTUs are going to be bouncing around and VLANs get their MTUs changed then perhaps a VLAN needs both a desired and actual MTU setting. The VLAN's interface would then be the minimum of the desired and actual MTU. I suppose it isn't too unlike having both an administrative (desired) and operational (actual) interface state. rick jones