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speed/duplex settings

Inderjeet Singh


Can experts here guide on two queries below..

01: I know only speed mismatch causes the connection lose.. but if two ends are configured at auto/auto and I just set duplex at one end as "full" (speed remains auto/auto).. will this cause connection lose ??

02: Is it require to change the speed first and than duplex.. ??
      If I change the duplex first, will it cause the connection lose ??

4 Replies 4

Junaid Shah

01. It wont cause the connection to be lost but with WAN connections i have seen performance degradation such as packet loss on my next-hop IP.

02. Duplex will not cause the connection to be lost


You can run both commands quickly in few seconds. It is up to you which order you choose for duplex and speed but both needs to be synced and same at both end for optimal performance. Good Luck.

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#1 If one side is configured auto duplex and the other side full duplex, the connection will normally still pass traffic but very slowly due to the errors caused by the duplex mismatch (the auto side will set itself to half duplex).


#2 I believe some equipement wants one setting set before the other, and other equipment doesn't care.  I recall changing the duplex setting may cause the interface to bounce.



If you equipment works with auto/auto, use it, that's current best practice (for the 21th century wink).

Helo friends,

Thanks for helping .. Your inputs are really helpful..

I would like to share my experience when I did this activity.. Just to mention, this was changed on WAN Ethernet links termination as RJ45 connection on CE router..

01: Initially the supplier and CE router was set on auto/auto..  I check on CE, the interface was negotiating on 100M/half duplex..  I just changed the duplex on CE .. and I got disconnected.. to me I should not be since duplex mismatch doesn't cause lose of connection.. so my end was speed auto, duplex full and supplier is auto/auto.. this looks still perfect.. and should not cause any disconnection.. but it happened..

02: When I asked my colleagues here, they said  yes.. speed should be changed first before duplex.. I have not read it anywhere but practical implmentation can be different.. so couldn't understand the logic of first changing the speed.

I got to reboot the router to get the connection back.. and worst of my bad luck, it was a Data Center router... sad


1:  Duplex mismatch causes one side to be half duplex, the other to be full.  You'll see late collisions on the half-duplex side if the full-duplex side transmits when the half-duplex side thinks it's in the clear.  The full-duplex side may see input errors during times of collision, but the collision counters won't increment since collision detection is turned off.

2:  Changing duplex will not cause a bounce.  However, changing speed will.  It will be momentary, but it will be there.  If you have a routing relationship on the link, expect it to bounce.

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