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What happens when I do not turn on Auto Negotiation, force my MAC and PHY as 100Mbps, but instead connect (say by mistake) to a 1000Mps network (say all devices in that LAN runs at Giga Speed)?

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Normally, both ends would just negotiate the highest available speed.

But if you have autonegotiation disabled, but the switch expects it to be enabled, you'll be stuck with a very unreliable 100 Mbps half-duplex connection.

Parallel detection is used when a device that is capable of autonegotiation is connected to one that is not. This happens if the other device does not support autonegotiation or autonegotiation is administratively disabled. In this condition, the device that is capable of autonegotiation can determine and match speed with the other device. This procedure cannot determine the presence of full duplex, so half duplex is always assumed [by the autonegotiating switch].

Which leads to...

The end result is a connection that is working but performs extremely poorly because of the duplex mismatch. Symptoms of a duplex mismatch are connections that seem to work fine with a ping command, but "lock up" easily with very low throughput on data transfers; the effective data transfer rate is likely to be asymmetrical, performing much worse in one direction than the other.

In either case, other devices connected to the same switch will not be affected, since each port of a switch is configurable separately (unlike hubs, which just link all ports to one large network).

See autonegotiation and duplex mismatch on Wikipedia.

Personally, I think disabling autonegotiation causes more trouble than gain. Does saving a tiny bit of battery matter when you waste more by having transfers take ten times longer, and even more when the mismatched connection truly sucks?

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+1 for 'splainin' half duplex. – goblinbox Feb 13 '12 at 17:53

It would work as if you connected a FastEthernet device to a GigaEthernet switch. Also, you won't bring the whole network speed down to 100Mbps, everything else will still work at 1Gbps.

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It should negotiate a 100Mbps speed. However, some devices may have issue. You will need to test with your hardware

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But can it negotiate if the device has not turned it ON? – Prajagop Feb 12 '12 at 17:49
@Prajagop As long as your Switch can negotiate down to 100Mbps you are fine. – Karlson Feb 12 '12 at 17:50
@Karlson: But the point is that the switch cannot negotiate down to 100 Mbps, because the device will refuse to reply to negotiation messages. – grawity Feb 12 '12 at 17:58

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