Auto Negotiation - Yes or No?

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Jan 23rd, 2008
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For a long time i have religously hardcoded speed and duplex on all my switch to switch and switch to server connections. However, I have read some info on the Sun site that seems to suggest the is way of working is outdated and that Network technologies are mature enough now to allow the network admin to configure auto\auto. They even suggest that in some instances hard coding speed and duplex can cause the incorrect detection of downed interfaces causing problems with STP.

What do you think?

Shold we start trusting auto negotiation??

Does anyone have any documentation from Cisco that recommends either method?



Correct Answer by Edison Ortiz about 9 years 6 months ago

I recommend you check out this URL:




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Overall Rating: 4.7 (3 ratings)
shrikar.dange Wed, 01/23/2008 - 03:56
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ASFAIK that manual config of 100 Mbps @ full duplex do not interact properaly with interfaces configd with auto.This is because as per RFC standard both interfces recuire to send auto-negoatiation parameters for link only when both ends are confgd for auto-neg.If one end is hard coded then there will b duplex mismatch resulting errors and resulting switch interface to operate in half duplex.

As cisco recomondes always manually confgd 10/100-Mbps fastethernet & 10/100/1000Mbps gig ethernet on both link ends for critical connections to servers or 3rd party equipment.

Use auto-neg for interconnecting cisco devices and user workstations.



shri :)

Joseph W. Doherty Wed, 01/23/2008 - 04:55
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Within Cisco's best practices for the large L3 switches (, they recommend for 100 Mbps "As a rule, first use autonegotiation for any type of link partner." but also recommend if there's a problem configure for 100/full. For gig, they have "Enable Gigabit negotiation on all switch-to-switch links and, generally, on all GE devices."

My experience has been with current gen equipment, there's often less issues with auto working correctly vs. someone forgeting to properly configure both sides of the connection expecially when for some reason they connect to a different port (like when dealing with a hardware failure).

glen.grant Wed, 01/23/2008 - 17:11
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I think you will see very few problems between cisco devices leaving them as auto . And unless you run into a specfic problem I wouldn't hardcode them . We run into very few problems for speed/duplex anymore and when it is it is usually because the "server guys decided to hardcode the speed /duplex because they fail to understand the implications of hardcoding . They think if they hardcode the server the switch will automatically adjust itself to 100/full and we all know that isn't true it will default to 100/half.

srue Fri, 01/25/2008 - 14:13
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there are also some 'special' cases with IP Phones and auto mdix technologies. Those situations are best left to auto-negotiate also.

serotonin888 Fri, 01/25/2008 - 22:55
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Thanks to everyone who responded.

I think the gerneral concensus is that auto between cisco devices is OK and works. And Auto between servers (and non-cisco devices) but use with caution.

In Edisons URL i picked up the follwoing statement.

"Because of these recent issues, the use of autonegotiation is regarded as a valid practice."




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