Autonegotiation Required for 1000base-T per IEEE standard

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Jul 26th, 2007
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Per the IEEE standard for 1000base-T, autonegotiation is a requirement.

On switches like the 6509 you can set the speed to 1000 and duplex to full on a gigabit port. My question is this, can anyone point me to some Cisco documentation that identifies what exactly this accomplishes? My assumption is that while you can "force" 1000/full, the port is still doing the autonegotiations required per the standard in order to establish the master-slave relationship and correctly establish timing control for the link. I'm guess that what this really does is do a "gigabit-or-nothing autonegotiation." Thanks for the help.

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lamav Thu, 07/26/2007 - 09:26
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Im not sure what you are asking. What is your question? request?

mlsommer Thu, 07/26/2007 - 09:55
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OK, let me try to clarify.

On a 6509, you can set a gig port's speed to 1000. "(config-if)#speed 1000"

What does this actually do? Per the standard:

Autonegotiation is a requirement for 1000base-T. So, that being said, does this configuration command allow you to set the port to something that is outside of the standard or is the port still autonegotiating the link?

lamav Thu, 07/26/2007 - 16:19
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First, not every gig port is configurable. For example, the 16-port 1000BaseX Ethernet fiber interface card is NOT configurable. Interfaces on it can only be set for 1000 and full duplex. Changing it is not even a choice.

Now, that having been said, autonegotiation entails more than just speed and duplex. It involves an exchange of interface capabilities, session negotiation and other parameters. so, you CAN be hard-coded for 1000/Full and STILL be autonegotiating.


mlsommer Thu, 07/26/2007 - 17:41
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Yeah, that's what I assumed based on the standards document, but I can't find anything on CCO that spells out exactly what is and is not taking place on a 1000base-T interface when configured for autonegotiation vs. forced 1000/full. Maybe nothing of the sort exists. I'm just having trouble convincing coworkers without that all-authoritative Cisco documentation.

RAPHAEL KRUCZKOWSKI Fri, 07/27/2007 - 04:04
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I know this will not help you in Cisco-land. But with Foundry switches the command is "speed-duplex 1000-full-master" or "slave". I have tried looking for this answer with Cisco also and got nowhere.

bkupiec Thu, 10/11/2007 - 07:47
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Have you found any further information from Cisco? I'm also interested in the answer. Thanks.

Cathal Mooney Thu, 07/18/2013 - 06:38
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To the best of my knowledge issuing these commands will not prevent the port engaging in the autonegotiation process, however it will only advertise itself as being capable of operating at 1000Mb at full-duplex, rather than also saying it can do 10/100 and half-duplex aswell.


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