cannot set mtu > 1500 on em0 optical Gig-E

[Jack Vogel]

Ahem, its our OLDEST 1G part, it was ancient when I took this job 5 years
ago :)
And the code is working according to spec, jumbos are not supported on the
device,
that's not the same as saying its not possible.

We only have one or two of these devices laying around, and I get a chuckle
any
time I've asked to use one to repro something, adapters aren't that
expensive,
time to think about an upgrade :)

Jack


On Thu, Feb 10, 2011 at 2:04 PM, Jay Borkenhagen <jayb at braeburn.org> wrote:

> 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
>