Date: Tue, 28 Oct 2008 14:23:50 -0700 From: John-Mark Gurney <jmg@funkthat.com> To: Nikolay Denev <ndenev@gmail.com> Cc: freebsd-net@freebsd.org, Jack Vogel <jfvogel@gmail.com> Subject: Re: ifconfig em0 mtu 9000 does not update the routing table Message-ID: <20081028212350.GF51033@funkthat.com> In-Reply-To: <84671113-DA04-4EFA-8640-E9AD2F09BB34@gmail.com> References: <DE4AF9A2-8741-4FC2-8E39-1F72A61FE8C4@gmail.com> <2a41acea0810280941g6c9846y6e0d4b8b19313e0a@mail.gmail.com> <84671113-DA04-4EFA-8640-E9AD2F09BB34@gmail.com>
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Nikolay Denev wrote this message on Tue, Oct 28, 2008 at 19:54 +0200:
> On 28 Oct, 2008, at 18:41 , Jack Vogel wrote:
>
> >It can't change the route table when you've given it no address:
> >
> >IE. ifconfig em0 HOSTNAME mtu 9000 will update it just fine.
> >
> >Cheers,
> >
> >Jack
> >
> >
> >On Tue, Oct 28, 2008 at 6:15 AM, Nikolay Denev <ndenev@gmail.com>
> >wrote:
> >Hello,
> >
> >As the subject says, I'm trying to enable jumbo frames on running
> >machine by setting
> >ifconfig em0 mtu 9000 by hand, but nothing changed, and I've fount
> >that when I list
> >the routing table with the MTU column it shows the connected routes
> >still with MTU 1500.
> >Is this supposed to work this way? I understand that making ifconfig
> >touch the routing table is ugly hack, but maybe the routing code can
> >be notified for interface changes by some other mechanism?
> >
> >btw, this is on 7-STABLE with if_em(4) interfaces.
> >
> >--
> >Regards,
> >Nikolay Denev
> >
>
>
> Hi Jack,
>
> Yes, I understand that, It just looked logical to me to update the
> existing entries (directly attached) if no address is given, so one
> can use the mtu knob alone.
>
> Anyways, one rarely touches the mtu alone and this not a real problem,
> I've just tried to enable jumbo frames on two running machines with
> just the ifconfig em0 mtu 9000 line and wondered why it does not work
> for a minute.
But what happens if you are talking to a machine that doesn't have
jumbo frames enabled? You'd loose communications with the machines
if ifconfig auto updated...
The real missing part of the puzzle is a daemon that sits around and
probes neighbor's MRU size and updates each host route w/ the size
that is supported by that host...
/me has want to write such a daemon, but the work to do it in C is time
consuming.
--
John-Mark Gurney Voice: +1 415 225 5579
"All that I will do, has been done, All that I have, has not."
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