ifconfig em0 mtu 9000 does not update the routing table

[John-Mark Gurney]

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 at 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."