[c-nsp] The myths of autonegotiate vs forced

[Jim Getker (getker)]

Well one problem with that is that a port configured to auto-negotiate
can only link up in half duplex the link partner is configured as fixed
speed.  This is called parallel detection.  The phy is transmitting fast
link pulses (FLP's). On the receive side it can detect FLP's, MLT3
coding for 100mbps, or Manchester for 10mpbs.  If the phy receives MLT3
or Manchester it links up the appropriate speed and half duplex, this is
required by 802.3.  If a port can only do auto-negotiation it can never
come up in full duplex when it parallel detects.  So if a link partner
does not support auto-negotiation but it does run full duplex you could
never fix the duplex mismatch.  I don't know if this was the original
intent behind the way the speed control works, but it would be an issue.

Regards,

Jim 



-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of John Neiberger
Sent: Friday, August 20, 2010 9:38 AM
To: Mikael Abrahamsson
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] The myths of autonegotiate vs forced

On Fri, Aug 20, 2010 at 2:18 AM, Mikael Abrahamsson <swmike at swm.pp.se>
wrote:
> On Fri, 20 Aug 2010, Peter Rathlev wrote:
>
>> On Fri, 2010-08-20 at 09:34 +0200, Mikael Abrahamsson wrote:
>>>
>>> In case of having to force, do use "speed auto / duplex full" if
your
>>> equipment supports it.
>>
>> And on a side note, if one needs to provide less-than-linerate speeds
>> (e.g. a 100 Mbps customer on a gigabit interface) it's much better to
>> use "speed auto 100" than "speed 100". Not all devices/modules
support
>> it (by far), but it will give you the benefits of autonegotiate while
>> still "locking" the interface at a lower speed.
>>
>> The 3560G supports it, most 6500 modules seem not to AFAICT. Let's
see
>> if 100 Mbps disappears before this will be generally implemented.
>
> I feature requested this (why stop doing autoneg just because you
select a
> single speed/duplex, there is no downside to still announcing your
> capabilities), the response back was the usual "you're the only one
asking
> for this"-answer.

In my opinion, this is how things should behave by default. Fast
Ethernet devices often check to see if they have an autonegotiating
link partner and will fall back to half duplex if they don't detect
one. For the life of me I can't understand Cisco's reasoning for
disabling it completely when the settings are manually configured. It
costs you nothing to leave Nway enabled, and it regularly causes
problems when it is disabled. What could their logic possibly be?
There are two options and they picked the worst one. I can't think of
any rationale that holds up to reasoning.
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