[rescue] RE: SunFastEthernet questions

Gavin Hubbard ghub005 at xtra.co.nz
Wed Sep 3 09:07:06 CDT 2003

Hi Curt

As I understand it, the problems between Sun HME cards and Cisco switches
stem directly from autonegotiation.

Some types of NIC will always check for an autonegotiating partner while
other NICs do not. In particular, Sun decided it would be a good idea for
the HME NIC to check for an autonegotiating partner regardless of the speed
and duplex settings. The HME NIC will reset itself to half duplex if it does
not detect autonegotiation on the wire. Unfortunately when you check the
speed and duplex settings on the card it will state it is running at
100/full :-/

Older workgroup switches like the 2924 behave like the HME. Even if you
manually set the speed and duplex on each end of the link, both devices
still participate in autonegotiation (but they only offer the speed and
duplex they were configured for to the other device). This means you end up
with both devices happily talking to each other at 100/full.

However the modern Cisco switches do not do this. In particular, the
autonegotiation is disabled completely if you manually set the speed and
duplex. If you set the switch to 100/full it will only do 100/full and
nothing else. If you attach an HME to that port and you manually set the HME
to 100/full the NIC still checks the link for an autonegotiating partner.
When it doesn't detect one it makes the faulty assumption that full duplex
is not possible and it falls back to half duplex.

Unfortunately the only method for setting speed and duplex mentioned in the
Fast Ethernet standard is through autonegotiation (when autonegotiation is
not used, the default behaviour for NICs is unspecified). There are
basically two common behaviors among fast ethernet devices when you disable
autonegotiation and the real problems occur when you have a mix of devices
with different philosophies.

Does this answer your question?




So,  let me make sure I understand you...

Even if my hme and my autonegotiated switch negotiate at 100baseTX full
duplex (and never waver from that), the fact that the hme is in auto
negotiate is going to degrade my performance ?

This doesn't make sense to me (I'm not saying that you are wrong,
just that it doesn't make sense).  What could possibly be cuasing
a degradation in performance if negotiation has locked in at 100 full
and stayed at 100 full (with no error rate).

I also don't know the performance in my case of the Solaris for Intel
drivers for the ethernet card in my Solaris for Intel server (3COM 3C905).

I've heard the older cards (Big Mac (bmac)) had less performance (at
least under Solaris).

-- Curt

More information about the rescue mailing list