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I ran across this idea today and had conflicting opinions on this from various co-workers.

I am trying to understand if the communicating devices' auto negotiation settings need to match each other when communicating over a switch which has the ports configured to correctly match the connected devices auto negotiation settings. I cannot find an example explicitly stating whether this is OK or not.

I was arguing the communicating devices' auto negotiation configuration needs to match, but cannot back it up with clear examples and suspect I am in fact incorrect. Some documentation on the subject appears to suggest that only the link to the switch matters but is not clear.

Scenario:

All devices would be configured to 1000/Full Duplex. Linux if that matters.

  • Device 1 and Switch 1 Port 1 are connected and both switch port and device have auto negotiation On
  • Device 2 and Switch 1 Port 2 are connected and both switch port and device have auto negotiation Off
  • Device 1 and Device 2 send and receive data to/from each other over UDP or TCP.

Would this configuration cause communication issues for Device 1 and Device 2?

Paj
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  • Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. – Community Jun 17 '22 at 07:41

1 Answers1

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Auto negotiation enables nodes with different capabilities to establish a link. Each side advertises the link modes it supports and the best mutual mode (by IEEE 802.3 appendix 28B.3) is chosen.

There aren't any auto negotiation settings - apart from some devices featuring limited advertisement, e.g. Auto-1000 on a 10/100/1000 port (advertise 1 Gbit/s only). Of course, a Fast Ethernet port (10/100) cannot link with such a port, lacking any mutual mode.

Unless you have a very good reason and there's no other solution, you need to have auto negotiation active at all times. Especially when forcing a specific mode on one side only, the most likely outcome is a duplex mismatch. Also, with Gigabit Ethernet and faster auto negotiation is required.

Zac67
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