Re: Forcing 100FD

+++ Andrea Carpani [26/10/05 11:24 +0200]:

> There are a lot of comments about
> autnoeg being good and forcing bad, but I found no explanation on why.

Forcing one side and leaving the other side in various non-forced modes
leads to unexpected effects. Cisco has a nice diagram about the results:

http://www.cisco.com/warp/public/473/46.html#auto_neg_valid

for instance:

Forcing your NIC to 100Mbps Full Duplex and leaving the switch to auto
results in a flapping link in most configurations. If the switch is
connected to a large network, has STP enabled and STP is not specifically
(read: correctly) configured the resulting topology-changes will hinder
performance in places you won't expect.

I've seen this scenario in two buildings, each with a couple of hunderd
PC's, interconnected by a slow WAN-link. Flapping links and the resulting
STP BPDU's saturated the 2.3Mbit interconnection. After some reconfiguring
this flood of traffic simply disapeared. The in-house admin declared it
magic. He wasn't aware of STP and the basics of ethernet, and was
considering an expensive fiber between the two buildings. Traffic over the
link was mostly email, and it's hard to fill 2.3Mbit/s with mail, even in a
windows-world. Enabling auto-config company-wide, filtering STP and setting
two STP-roots was all I did for a handsome consulting-fee.

Autonegotiation works. If you don't get the link you're expecting its a sign
of trouble of some kind (mostly cabling) which should deserve your attention.
Forceing a higher speed than autonegotiation will often result in errors on
your link, which can be harder to detect than a simple fallback to 10Mbps.

-- 

Ruben
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