Setting the auto-neg speed of an ethernet copper link at a cisco switch

In some cases it could be interesting, that I am able to limit my port speed. For example if I want to summarize my traffic on an uplink. It could be better to have the end devices configured with 100 MBit/s so that 10 end device can use a 1 GBit/s uplink without any problems. So a way to archive this goal might be using fixed values for link speed and  duplex. But if I do so I must be sure that the end device and the switch have been set to the same values. Because if I make a mistake with that it could end very slow!  This also I will  call administration made by sneakers. So this is normally not what I like to do. Also it is often not possible to administrate every client because they do not belong to me.

So for that kind of reason, it would be better, to use the benefits of autonegotiation here, too. And today the most end devices uses auto negotiation by default, because when they want to communicate at 1GBit link speed auto negotiation is MANDATORY. But now we want to reduce the link speed.

So Cisco switches do support this by the following commands.

Switch(config-if)# speed auto 100
Switch(config-if)# duplex auto

or

Switch(config-if)# speed auto 10 100
Switch(config-if)# duplex auto

Further information can be found here:

http://www.cisco.com/c/en/us/support/docs/lan-switching/ethernet/10561-3.html#when

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