From mboxrd@z Thu Jan 1 00:00:00 1970 From: Patrick McHardy Subject: Re: [PATCH] vlan: propogate MTU changes Date: Tue, 07 Oct 2008 01:02:08 +0200 Message-ID: <48EA98F0.40302@trash.net> References: <20081006173024.2741cc01@speedy> <48EA369F.3090306@trash.net> <20081006195446.1dc5a372@speedy> <48EA9223.8090700@trash.net> <48EA964A.6060503@hp.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Cc: Stephen Hemminger , "David S. Miller" , netdev@vger.kernel.org To: Rick Jones Return-path: Received: from stinky.trash.net ([213.144.137.162]:56832 "EHLO stinky.trash.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752722AbYJFXCP (ORCPT ); Mon, 6 Oct 2008 19:02:15 -0400 In-Reply-To: <48EA964A.6060503@hp.com> Sender: netdev-owner@vger.kernel.org List-ID: Rick Jones wrote: > Patrick McHardy wrote: >> 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? The traffic actually shouldn't stop since the MTU isn't enforced by the lower layers and also usually not by the driver. So I feel unable to make a policy decision when both views don't seem unreasonable. Especially given the fact that the "more suprising" behaviour so far has been our default. > 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. Thats assuming that the VLAN device is actually restricted by the ethernet device settings. I don't know if its always not the case, but I'm pretty sure it usually isn't. Which means there's no real need for an operational state wrt. MTUs.