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New Member

Gigabit Ethernet Duplex Negotiation

Hi Friends,

I am looking for some information on duplex modes on a Gig thernet port. I have read in some documents that Gig Ethernet supports only full duplex mode. I tried to verify this on my Core switch which is s stack of 3750 copper and fibre switches. On the copper Gig Ethernet ports I am able to execute the commnad Duplex Half or Full under the Gigabit ethernet configuration mode. However under the Fiber SFP connected interface this command is not supported. Below is the show interface status for both these links.

Port      Name               Status       Vlan       Duplex   Speed  Type

Gi1/0/1                          disabled     1            auto     auto     10/100/1000BaseTX

Gi2/0/1                          connected   trunk      a-full    a-1000 1000BaseSX SFP

Can some one throw some more light on fact how this works. Also since the fibre interface shows "a-1000" does this mean that if the far end device is negotiating to auto will the Fibre port negotiate to auto?.

Thaks in Advance !!!

Ragards

Umesh Shetty


  • LAN Switching and Routing
1 ACCEPTED SOLUTION

Accepted Solutions

Gigabit Ethernet Duplex Negotiation

It is true that Gig Ethernet is only defined as full duplex.  In terms of fibre, it is clear why that should be - the transmit and receive paths are independent and point-to-point, so it is inherently full duplex.  In terms of 1 Gbps copper, the paths are also independent - in fact the transmission and reception happen along all four pairs in both directions at the same time, due to a clever technique of balancing out your own transmitted signal.

Your 10/100/1000 ports, on the other hand, do support half-duplex, because half duplex is defined in the 100 and the 10 standards.  It is defined in the 10 Mbps standard because at the time there were hubs rather than switches, and therefore the media were shared, so half duplex was a must. By the time 100 Mbps came along hubs were diappearing, but still supported.  In fact, both 10 and 100 can be used in half-duplex (i,e, shared-media) mode, or full-duplex (point-to-point) mode..  Both 10 Mbps and 100 Mbps use only two pairs of the cable: one pair for transmit and one for receive.

The 10 Mbps copper standard uses a carrier of 20 MHz, while the 100 Mpbs uses a carrier of 125 Mz.  The clever clever 1000 Mbps standard also uses a carrier of 125 MHz, and gets the requisite number of bits by using all four pairs in both directions at the same time, then doing some clever 5-state encoding on each pair.  It also has autonegotiation of which pair does which job, so a cross-over cable and a straight-through cable are the same thing.

The CLI table ... the "a" means it is autonegotiating, and the 1000 is the actual speed the ends have agreed upon.

Hope this helps

Kevin Dorrell

Luxembourg

5 REPLIES

Gigabit Ethernet Duplex Negotiation

It is true that Gig Ethernet is only defined as full duplex.  In terms of fibre, it is clear why that should be - the transmit and receive paths are independent and point-to-point, so it is inherently full duplex.  In terms of 1 Gbps copper, the paths are also independent - in fact the transmission and reception happen along all four pairs in both directions at the same time, due to a clever technique of balancing out your own transmitted signal.

Your 10/100/1000 ports, on the other hand, do support half-duplex, because half duplex is defined in the 100 and the 10 standards.  It is defined in the 10 Mbps standard because at the time there were hubs rather than switches, and therefore the media were shared, so half duplex was a must. By the time 100 Mbps came along hubs were diappearing, but still supported.  In fact, both 10 and 100 can be used in half-duplex (i,e, shared-media) mode, or full-duplex (point-to-point) mode..  Both 10 Mbps and 100 Mbps use only two pairs of the cable: one pair for transmit and one for receive.

The 10 Mbps copper standard uses a carrier of 20 MHz, while the 100 Mpbs uses a carrier of 125 Mz.  The clever clever 1000 Mbps standard also uses a carrier of 125 MHz, and gets the requisite number of bits by using all four pairs in both directions at the same time, then doing some clever 5-state encoding on each pair.  It also has autonegotiation of which pair does which job, so a cross-over cable and a straight-through cable are the same thing.

The CLI table ... the "a" means it is autonegotiating, and the 1000 is the actual speed the ends have agreed upon.

Hope this helps

Kevin Dorrell

Luxembourg

New Member

Gigabit Ethernet Duplex Negotiation

Hi Kevin,

Thanks for your Image

What you said in the third paragraph, has really generated a lot of interest in knowing more about it and in detail. I know it won't be possible for you to type everything in detail, so could you please share any dosuments that would help me gain more on this topic. In case I have any further queries I will probably need your help..

Thank you again for the explanation and help.

Regards

Umesh Shetty

Hall of Fame Super Gold

Gigabit Ethernet Duplex Negotiation

Can some one throw some more light on fact how this works. Also since the fibre interface shows "a-1000" does this mean that if the far end device is negotiating to auto will the Fibre port negotiate to auto?.

Fibre optic is a very bad example because it will ALWAYS negotiate to FULL duplex and ALWAYS negotiate to the modules' set speed.

New Member

Gigabit Ethernet Duplex Negotiation

Hi Leo,

How can we set the speed of the module ? is there a configuration command for this?

Regards

Umesh Shetty

Hall of Fame Super Gold

Gigabit Ethernet Duplex Negotiation

How can we set the speed of the module

ALL Cisco fibre optic modules are single speed.  You can't change the speed nor can you change the duplex setting.

So if you have a GLC-SX-MM on one side and you have a GLC-GE-100FX on the other side, the link won't come up until you replace either end with an identical speed.

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