[c-nsp] Why hard-setting speed and duplex on Fast Ethernet is bad

[Heath Jones]

Thats a very good point John!

Any thoughts why a cat5 <5m non-erroring link with auto on both ends that
negotiates 100/full would sometimes (once,twice per week) drop down
(10/half) and then back up again shortly after?
Forcing both ends 'fixes' it (stops the flaps).. Seen that one before?

Cheers




On 20 August 2010 14:46, John Neiberger <jneiberger at gmail.com> wrote:

> Someone in the other thread mentioned that they were surprised to see
> that so many people were against manually configuring speed and
> duplex, so I thought I would explain why this is a bad idea most of
> the time.
>
> The Fast Ethernet standard does not mention how devices are supposed
> to behave when manually configured. It only deals with Nway
> autonegotiation. The problem is that there are two possible behaviors
> when settings are manually configured:
>
> #1:  Participate in Nway, but only offer the configured settings
> #2:  Disable Nway entirely and run at the configured settings
>
> Cisco's older switches, like the XL series, used behavior #1, as do
> most PC/Server NICs that I've run across for that past eight years or
> so. Beginning around the time the 2950s came out, Cisco decided to
> switch to behavior #2. If you connect two devices that use behavior
> #1, you'll be fine. If you connect two devices that use behavior #2,
> you'll be fine. But what happens if you connect a "#1" device to a
> "#2" device? You get a duplex mismatch! The device that still
> participates in Nway is going to expect to see an autonegotiating link
> partner. When it doesn't detect one, it follows the standard and
> assumes it is connected to a hub or some device that can't do full
> duplex and it falls back to half duplex, often without telling you.
> This creates input errors on the full duplex side and late collisions
> on the half duplex side. If you have hard-set your speed and duplex on
> a Cisco switch and you're seeing a lot of input errors, you likely
> have a duplex mismatch because of this problem. Setting BOTH sides to
> auto usually resolves this issue.
>
> Auto is the only reliable way to go these days unless you know ahead
> of time which of the two behaviors your devices are choosing. If you
> don't know, autonegotiation is the most likely way to get a good
> connection at 100/full.
>
> I hope that helps clear some of that up.
>
> -John
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