cannot set mtu > 1500 on em0 optical Gig-E

[Jay Borkenhagen]

Jack Vogel writes:
 > Thanks Sergey :)
 > 
[...]
 > >
 > > Hmm.. Did jumbo frames really work on these cards for you? Intel
 > > claims: "Adapters based on the Intel® 82542 and 82573V/E
 > > controller do not support Jumbo Frames. These correspond to the
 > > following product names:
 > >
 > >    * Intel® PRO/1000 Gigabit Server Adapter
 > >    * Intel® PRO/1000 PM Network Connection "
 > >
 > > As seen from code, the behavior was changed to match the
 > > description ("keep old 82542 from using jumbo frames") and merged
 > > to 6-STABLE before 6.3.
 > >


Well, I've got tcpdumps showing DF-bit-set pings succeeding to and
from such a NIC with MTU >> 1500, on a box running 6.2-RELEASE.
(pciconf data below.)  In this particular case I have MTU=4470, which
is really good enough for what I need, and DF pings succeed up to
'ping -s 4442'

Maybe Intel thought they couldn't handle MTUs approaching 9k, or maybe
they had problems approaching linerate -- neither of which are
concerns in my application...


em0 at pci0:2:0:   class=0x020000 card=0x10008086 chip=0x10008086 rev=0x03 hdr=0x00
    vendor   = 'Intel Corporation'
    device   = '82542 Gigabit Ethernet Controller'
    class    = network
    subclass = ethernet