Date: Sun, 22 Jul 2007 20:09:43 +0200 From: Sten Daniel Soersdal <netslists@gmail.com> To: karels@karels.net Cc: freebsd-net@freebsd.org, Artyom Viklenko <artem@aws-net.org.ua>, Artem Belevich <fbsdlist@src.cx>, Julian Elischer <julian@elischer.org> Subject: Re: 6.2 mtu now limits size of incomming packet Message-ID: <46A39D67.5030900@gmail.com> In-Reply-To: <200707211702.l6LH2ukt039317@redrock.karels.net> References: <200707211702.l6LH2ukt039317@redrock.karels.net>
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Mike Karels wrote: >> Any two hosts, connected to single Layer2 network MUST use >> same MTU. Any other cases lead to hard-to-solve problems. > > I'd have to disagree. In fact, I'd say that any two hosts on the > same L2 network must use the same MRU. In particular, if a host > choses to use a lower MTU, if that also lowers the MRU, *that* is > the cause of interoperability problems. > > David DeSimone <fox@verio.net> wrote: > } You are correct about misconfigured networks. In my experience, > } the only reason to ever reduce the MTU is to work around a problem > } discovered in someone else's network (not my local segment). Fixing > } the problem by getting someone else to fix their network is generally > } too hard. If MTU == MRU was forced behavior, the viability of this > } workaround would be removed, one less tool in the toolbag, so to speak. > > Exactly. In our local labs, we also reduce the MTU to test PMTU discovery. > Requiring MRU == MTU makes this more difficult. True, it's a contrived > situation, but as you say, one less tool in the toolbag. > > Mike To leave this tool in the toolbag one could set interface mtu to the upperbounds of what one needs (e.g. 1500) and use pr route --mtu setting to work around those broken networks. That shouldn't break the inbound oversized frames (frames smaller than 1500) but lower the outbound mtu. -- Sten Daniel Soersdal
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