Speed and Duplex Settings

Cisco Ethernet Interfaces are set to auto negotiation for their speed and duplex settings by default, which means that both devices on a link will automatically negotiate the best settings for their connection. They will first share their speed and duplex capabilities and then decide on which ones to take.
If auto negotiation is not active on both sides, the interfaces will find their speed settings based on the incoming electrical signal, as long as the fixed sides uses a speed which is supported by the other side.

So basically we could set the speed on one side to fix and on the other side to auto, even though its not a good practice, it is still possible.


The Lab layout is a simple Router (Cisco2821) to Switch (WS-C3750) connection.

Both interfaces (gig) are set to auto at the beginning:

SW1#sh int gig1/0/1 capabilities
GigabitEthernet1/0/1
 Model:                 WS-C3750G-24T
 Type:                  10/100/1000BaseTX
 Speed:                 10,100,1000,auto
 Duplex:                half,full,auto

SW1#sh int gig 1/0/1
GigabitEthernet1/0/1 is up, line protocol is up (connected)
 Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX

Afterwards the speed on the router is set to 10 and subsequently increased to 1 Gig, the duplex is left at auto on both sides:

Router(config)#int gig 0/0
Router(config-if)#speed 10

SW1#sh int gig 1/0/1
GigabitEthernet1/0/1 is up, line protocol is up (connected)
 Full-duplex, 10Mb/s, media type is 10/100/1000BaseTX

Router(config-if)#speed 100

SW1#sh int gig 1/0/1
GigabitEthernet1/0/1 is up, line protocol is up (connected)
 Full-duplex, 100Mb/s, media type is 10/100/1000BaseTX

Router(config-if)#speed 1000

SW1#sh int gig 1/0/1
GigabitEthernet1/0/1 is up, line protocol is up (connected)
 Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX

As you can see, the switch automatically adjusts its speed settings even though, the router side is fixed. If the switch would only support 10/100, the link wont come up if the router is fix set to 1 Gbit/s.
In contrast to the speed settings, the duplex settings can only be found over auto negotiation, if auto negotiation is disabled the interface will automatically use its default settings for the speed value.
The defaults are as follows:

10 Mbit/s   half duplex
100 Mbit/s half duplex
1 Gbit/s     full duplex

This also explains, why mismatched auto negotiation on 10 and 100 Mbit/s links use half duplex and cause a performance degradation on that link.

Even though it is possible to play with those settings, it is still considered to be best practice to set the interfaces to auto negotiation.

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One comment

  1. reaper81

    Good post. I used to be very confused when I saw links using 100 Mbit but half duplex but once you understand autonegotation and FLP it all makes sense 🙂

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