Unanswered Question
Jun 29th, 2007


What happens when both the end(pc and switch) is configured as autonegotiation ?

I have this problem too.
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Jon Marshall Fri, 06/29/2007 - 03:23


If both ends are configured to auto-negotiate they should negotiate to the highest common denominator between them ie. if the pc is 10/100 capable and the switch port is a 10/100/1000 capable you would expect the PC to negotiate to 100Full.



mohammedmahmoud Fri, 06/29/2007 - 03:28


When auto-negotiation is configured the speed is negotiated to the highest speed using the Fast Link Pulses (FLP), and the duplex is also negotiated, however, if auto-negotiation is disabled on either end of the cable, the switch detects the speed anyway based on the incoming electrical signal, but the devices without a configured/negotiated duplex setting must assume a default. Cisco switches use a default duplex setting of half duplex (HDX) (for 10-Mbps and 100-Mbps interfaces) or full duplex (FDX) (for 1000-Mbps interfaces), which can introduced duplex mismatch, example if one side is auto and the other is full, the auto side will assume half duplex, which introduces duplex mismatch.

Accordingly as a best practice, either to make both sides Auto, or configure both sides explicitly to avoid any Duplex mismatch problems.

[edit] Sorry Jon, didn't see your reply.


Mohammed Mahmoud.

Jon Marshall Fri, 06/29/2007 - 04:26

Hi Mohammed

No problem, it happens to me all the time and to be honest you gave a far better explanation than me.


mohammedmahmoud Fri, 06/29/2007 - 04:45

Hi Jon,

Thank you very much, its really my honor to support this forum with persons like your self.


Mohammed Mahmoud.

guruprasadr Fri, 06/29/2007 - 04:33

HI MohanRaj, [PLS Rate if HELPS]

1. NIC = AUTO [ 1000 Mbps, Full-duplex ]

Switch = AUTO [ 1000 Mbps, Full-duplex ]

Comments:Assuming maximum capability of Catalyst switch, and NIC is 1000 Mbps, full-duplex.

2. NIC = 1000 Mbps, Full-duplex [ 1000 Mbps, Full-duplex ]

Switch = Auto [ 1000 Mbps, Full-duplex ]

Comments:Link is established, but the switch does not see any autonegotiation information from NIC. Since Catalyst switches support only full-duplex operation with 1000 Mbps, they default to full-duplex, and this happens only when operating at 1000 Mbps.

3. NIC = 100 / 1000 Mbps, Full-duplex

Switch = 100 / 1000 Mbps, Full-duplex

Correct Manual Configuration

4. NIC = 100 Mbps, Full-duplex [No Link]

Switch = 1000 Mbps, Full-duplex [No Link]

Comment:Neither side establishes link, due to speed mismatch

5.NIC = 100 Mbps, Full-duplex [100 Mbps, Full-duplex]

Switch = Auto [100 Mbps, Full-duplex]

Comments:Duplex Mismatch 1 & Viceversa

6. NIC = 100 Mbps, Half-duplex [100 Mbps, Half-duplex ]

Switch = Auto [100 Mbps, Half-duplex ]

Comments:Link is established, but switch does not see any autonegotiation information from NIC and defaults to half-duplex when operating at 10/100 Mbps

7. NIC = 10 Mbps, Half-duplex [10Mbps, Half-duplex ]

Switch = Auto [10Mbps, Half-duplex ]

Comments:Link is established, but switch does not see Fast Link Pulse (FLP) and defaults to 10 Mbps half-duplex.

8. NIC = 10 Mbps, Half-duplex[ No Link ]

Switch = 100 Mbps, Half-duplex [No Link]

Comments:Neither side establishes link, due to speed mismatch.

9. NIC = AUTO [100 Mbps, Half-duplex ]

Switch = 100 Mbps, Half-duplex [100 Mbps, Half-duplex ]

Comments:Link is established, but NIC does not see any autonegotiation information and defaults to 100 Mbps, half-duplex.

10.NIC = Auto [10 Mbps, Half-duplex ]

Switch = 10 Mbps, Half-duplex [10 Mbps, Half-duplex ]

Comments:Link is established, but NIC does not see FLP and defaults to 10 Mbps, half-duplex.

NOTE:NIC= [....] = Resulting NIC Speed/duplex

Switch = [....] = Resulting Switch Speed/Duplex

I hope all combinations for Speed / Duplex Settings with Resulting Outputs is explained above. Hope this Helps you.


Best Regards,

Guru Prasad R


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