Force Port Speed 100-Full

James Fry
James Fry used Ask the Experts™
Greetings Experts, I'm hoping you can settle a point of dispute.  Is it possible to set a port to 1000-Full?  I read some documents that talk about it, but there are an equal number of documents that say that it's not possible.  A couple of sources say there is no such thing as half duplex when a port speed is set to 1000.  Any insight?
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Most Gigabit ports will auto-speed detect. and if the end to end performance between the fist connection point and the actual port will support 1000Mbps then it will show that it has settled on 1000MBPS speed.  Various factors will influence this. A bad or loose jack or plug , or a Cat5 cable that is not installed right will cause the 1000Mbps  speed to go down to 100Mbps.  If you force the speed to 1000Mbps  and the connection is not perfect, then there will be bit errors and retries to the point that  it may be much slower than if you leave it at 100Mbps


My understanding is you can set a port to anything you want that the gear supports.

However, if you do that that is the only speed it with run at.  So if you plug in something else that is not compatible with that speed, say you set it to 1gb and then plug in something that is 100x it won't work.  Or as DTH said, if the connection won't support it there is no fall back.

So while you might gain something in not having to negotiate the speed, it makes your network flexibility more of a bother if you need to plug in different equipment from time to time.  And when something doesn't work you might forget that you hard wired the port speed.
James FryEnterprise Solutions Architect


Understood.  I'm aware of the considerations and implications of hard setting ports.  Perhaps I should ask more specifically; is it possible to set a port to 1000-Full?  Or is it possible to set it to 1000-half?

Where this comes from, is I have a constituent that is up in arms because he doesn't have the option to hard set a port to 1000-Full.  It allows Auto-Negotiate up to 1000-Full of course, but there is a peering peice of equipment that he doesn't manage that they claim is set to 100-Full and says he has to match it for troubleshooting purposes.  However, several places document that there really is no such animal as hard setting a port to 1000-Full or 1000-Half because 1000Mbps is full duplex inherently.  Some even go so far as to say that half duplex isn't possible at 1000Mbps.  However, some documentation states the contrary.

Again, the question isn't why or whether a port should be forced, the question is is it possible to force that those particular speed/duplex settings.
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Is it possible - In a word - YES
Network Speed
As you can see on my Intel ProSet Network interface I can set a fixed 1000Mbps  but only in Full Duplex.
I don't believe that a gigabit interface can run in half duplex.

It depends on your equipment.

I just checked my Catalyst C2960 and it allows both half and full at all speeds, 10, 100, 1000.

You didn't say what you have, and I didn't ask.
James FryEnterprise Solutions Architect


Gotcha Gary, I'm curious, will it actually let you set the port at 1000-Half?  I only have access to a Procurve Switch and although it shows 1000-Full or Half as an option if you [?]:

Switch# config
Switch (config)# int e (1-24) enable
Switch (config)# int e (1-24) speed-duplex 1000-full
Switch (config)# end
Switch# wr mem

It gives this error if you actually try to execute the command

Value 1000-full is not applicable to port 43

Sorry not trying to belabor this question, and I really appreciate you guys giving your feedback.

Very curious.  I am using the Cisco Network Assistant to manage my switch.

I gives me an error when trying to set half 1000.

It also gives me and error when trying to set full 1000.

However, if I first set to full auto.  Then set 1000 it works but when I bring up the modify form the duplex option is missing and it only shows speed, and some other things.

Also, if I set auto 1000 it works but then the modify form also drops the duplex option.

Which means I can't go directly from 1000 to half 100 or half 10.

So it would appear that 1000 is always either auto or full, despite what the form says before applying it.
James FryEnterprise Solutions Architect


Thanks for confirming that for me Gary.  Those were my findings as well.  I'll leave the question open for a little while to see if anyone else has any related insight.  Sincerely, I appreciate you all taking the time to offer your comments on this.

My pleasure.

I had no idea it would act this way so I was interested in what it would do too.

On a related note,   The Broadcom BCM5790C nic in my R710 only lets me set 10 full and half, and 100 full and half, and auto.  This nic is capable of running  up to 10gbs.
Network Administrator
Is it possible to set a port to 1000-Full?

2960G(config-if)#speed 1000
2960G(config-if)#duplex half
Gigabit port is restricted to full duplex
2960G(config-if)#duplex full

As you can see above, the only option for the GB-port is full duplex
...... and yes, you can manually set a port to 1000-full

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