Autonegotiation

Why is it that the books insist that Auto-negotiation always works but in real world, it probably works at may be 70% at best especially where a 10/100 mbps port is concerned.

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Comments

broli720

What books have you read that state that?

spacenoxx

I suppose my wording is not accurate

But the book implies implies that.

Odom Cert Guide 100-101 Page 163/164,

"Disabling autonegotiation is not always a bad idea, but generally you should either use it on
both ends of the link or disable it on both ends of the link. For example, many network engineers disable autonegotiation on links between switches and simply configure the desired speed
and duplex on both switches. If enabled on both ends of the link, the nodes should pick the
best speed and duplex. However, when enabled on only one end, many issues can arise: The link
might not work at all, or it might just work poorly. "

In real life though they don't always negotiate best speed and duplex when auto is enabled on both ends.

broli720

Best practice is not to use it. I pretty much follow that rule of thumb so you don't run into duplex mismatches.

EdTheLad

Blessay: Autonegotiation on Ethernet - it works, it should be mandatory! - EtherealMind

spacenoxx

I thought I mentioned 10/100 categorically. And I am talking about a Cisco 2950G and a 3 year old Dell Optiplex 390 with a Giga interface. Nada... I had to set the entire lot of them on manual 100 and duplex at both ends. I can even make Video and post if needed.

I think the reason is IEEE says to fall back to half duplex if auto fails. Even a 10 Mbps full works better then 100 half if I am not mistaken.

EdTheLad

Rule of thumb, use autoneg if it works. If it doesn't work due to a mix match of legacy equipment, driver incompatibilities etc disable it on both sides. Books are not always correct, the author cant take into account all the different networking equipment thats available now, that was available 20 years ago and what will be available in 20 years.
You would sound very silly if tasked to design a core network and your argument to disable auto-neg throughout the network was due to your old Dell and 2950G not working properly.

spacenoxx

Try reading a message fully next time. Like where I said I had to change the "lot" implying "that"group of computers and not all.

Pupil

A lot of people have misconceptions when it comes to this topic. The book, Network Warrior explains this topic really well. Author suggests to use it for Gigabit Ethernet and not to use it for Fast Ethernet.

mella060

Generally, if its set to auto you auto not use it as Jeremy C says.