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Please help me on this case:

When I connect a NIC manually set to 100 Mbps and Full Duplex to a Gigabit Ethernet port (set to Auto/Auto), what will the result be?

Will the Gigabit Ethernet port negotiate to 100Mbps/FD or 100Mbps/HD?

Are there any documents that describe this behavior in detail?

Arima
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Thanh Quân
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By manually setting the speed and duplex on one side with automatic detection on the other side, the side with automatic speed will detect (not negotiate) the correct speed.

For the duplex, negotiation will fail. This results in the side with automatic duplex set to the default duplex for the speed. The default duplex for 1 Gb is FULL, and for 10 and 100 Mb is HALF.

Your scenario results in a duplex mismatch which causes several problems that will slow the link to a crawl.

Cisco has a nice table in this document: http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/17053-46.html

Apparently, someone has isolated the table in a separate document.

Ron Maupin
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There are quite a few different autonegotiation standards. The earlier standards (pre 802.3ab in 1999) were less than reliable, so you would have some cases where negotiation was done incorrectly between a host port and a switch port when both were doing negotiation.

The negotiation standards work by sending pulses, called FLP pulses, across the link. These pulses are 16 bits in most (all?) standards. The important bits are 0 through 12, 13, and 16. 0 through 12 defines the standards used and capabilities of the link. 13 defines a link failure and 16 defines an acknowledgement.

Most NICs won't send or pay attention to an FLP if autonegotiation is disabled in the NIC software. It is possible the port will come up in half duplex, depending on the switch (many early-generation 100Mbits switches were this way). On Gigabit capable ports, the port should default to full duplex, but this depends on how the manufacturer implemented the standard.

However, you didn't state whether or not your host was connected to a port capable of 100 Mbps operation. Not all Gigabit ports are (e.g., in a 1Gb/10Gb switch). If the port is connected to a switch which does not advertise the correct modes supported by your card, the port will not connect.

Here's an excellent reprint from PowerSolutions that describes the process in detail -- and this is a great presentation on autonegotiation from the IOL at University of New Hampshire which is an exceptional treatment of the subject.

Arima
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    "On Gigabit capable ports, the port should default to full duplex, but this depends on how the manufacturer implemented the standard." This statement is only true if the other side is also Gigabit. If the other side is 100Mbit, it should follow the FastEthernet standards and fall back to half-duplex. Otherwise nice post, so neither +/- from me. – YLearn Feb 13 '15 at 02:10
  • @YLearn, thanks, good comment. I had a hard time finding source material for this. It seems some chipset manufacturers who do 10/100/1000 will always default FD, and some HD. I know in Cisco land, it's almost always HD. – Arima Feb 13 '15 at 02:29
  • Thank Guys, After read all link from all of you. I found the answer. All device I use is Cisco so the result will be MISMATCH duplex as Ron Maupin said. I already tested with real switch and there is same result. Many thanks and Best regards, – Thanh Quân Feb 13 '15 at 06:43