Date: Thu, 13 Dec 2012 11:31:51 -0800 From: John-Mark Gurney <jmg@funkthat.com> To: Zaphod Beeblebrox <zbeeble@gmail.com> Cc: FreeBSD Net <freebsd-net@freebsd.org> Subject: Re: ipv6 route ignores MTU. Message-ID: <20121213193151.GE1563@funkthat.com> In-Reply-To: <CACpH0MdbHa%2BeYoE2XuaXkSUMOvkhL9bQkhFOgTtj1gOzbacQaw@mail.gmail.com> References: <CACpH0MdbHa%2BeYoE2XuaXkSUMOvkhL9bQkhFOgTtj1gOzbacQaw@mail.gmail.com>
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Zaphod Beeblebrox wrote this message on Wed, Dec 12, 2012 at 22:50 -0500:
> FreeBSD virtual.accountingreality.com 9.1-RC2 FreeBSD 9.1-RC2 #6 r241240:
> Sat Oct 6 01:43:35 EDT 2012
> root@virtual.accountingreality.com:/usr/obj/usr/src/sys/VRA
> amd64
>
> I have set a high MTU:
>
> em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 9014
>
> options=4019b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,TSO4,VLAN_HWTSO>
> ether 00:15:17:0d:04:a8
> inet x.y.z.q netmask 0xffffffe0 broadcast x.y.z.q1
> inet6 fe80::215:17ff:fe0d:4a8%em0 prefixlen 64 scopeid 0x5
> inet6 2001:1928:1::52 prefixlen 64
> inet 192.168.221.2 netmask 0xffffff00 broadcast 192.168.221.255
> nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
> media: Ethernet autoselect (1000baseT <full-duplex>)
> status: active
>
> curiously, although:
>
> [1:2:301]root@virtual:~> route -n get 192.168.221.84
> route to: 192.168.221.84
> destination: 192.168.221.0
> mask: 255.255.255.0
> interface: em0
> flags: <UP,DONE>
> recvpipe sendpipe ssthresh rtt,msec mtu weight expire
> 0 0 0 0 9014 1 0
>
> shows the correct MTU,
>
> [1:5:304]root@virtual:~> route -n get -inet6 2001:1928:1::84
> route to: 2001:1928:1::84
> destination: 2001:1928:1::
> mask: ffff:ffff:ffff:ffff::
> interface: em0
> flags: <UP,DONE>
> recvpipe sendpipe ssthresh rtt,msec mtu weight expire
> 0 0 0 0 1500 1 0
>
> shows the wrong MTU.
>
> The MTU in this case is set in the ifconfig_em0 rc.conf entry and the
> machine has been rebooted.
Are you sure that the mtu is set before the ipv6 address is assigned
to the interface? The route inherits what ever MTU was on the interface
when the route was created... It will be clamped if it is lower, but
will not be increased... This is so you can set the MTU on a route
(say you have a broken low memory device that only accept 512 byte packets)
and it will stay the way you set it..
--
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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