ipv6 route ignores MTU.

[Zaphod Beeblebrox]

On a:

FreeBSD virtual.accountingreality.com 9.1-RC2 FreeBSD 9.1-RC2 #6 r241240:
Sat Oct  6 01:43:35 EDT 2012
root at 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 at 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 at 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.