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Configure duplex and speed auto on SFP port

Answered Question
Sep 2nd, 2014
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Hello,

is it possible to configure "duplex auto" and speed auto" on an interface where a SFP module is inserted?

E.g. on an IE-3000-8TC with GLC-FE-100FX-RGD or Cisco WS-C3750G-12S with GLC-GE-100FX?

Thank you very much for your answer.

Kind regards

Kyle

Correct Answer by Peter Paluch about 2 years 11 months ago

Kyle,

I do not believe that you can change the transmission speed on the SFPs you're using. Their types clearly show that they are built for a particular variant of fiber Ethernet, namely, 100FX. As opposed to metallic Ethernet where a NIC can speak all standardized speeds from 10Mbps up to its maximum supported speed (e.g. a GigE NIC speaks 10, 100, and 1000Mbps), an optical Ethernet module usually support only the specific transmission speed for which it was constructed, and no other. The reason is fairly simple: different variants of optical Ethernet usually use different wavelengths, necessitating different hardware in SFPs.

So the lack of the speed command is natural.

Best regards,
Peter

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Kyle Lenderlof Wed, 09/03/2014 - 00:22
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Ohhh okay thanks.

What I am doing wrong here:

IE-3000-8TC  with IES-LANBASEK9-M Version 15.2(1)EY

interface GigabitEthernet1/2
 description trunk
 switchport mode trunk
 media-type sfp

!

Switch(config)#int gi1/2
Switch(config-if)#speed ?
% Unrecognized command
Switch(config-if)#duplex ?
  full  Force full duplex operation
  half  Force half-duplex operation

 

Thank you.

Correct Answer
Peter Paluch Wed, 09/03/2014 - 00:55
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  • Cisco Employee,

Kyle,

I do not believe that you can change the transmission speed on the SFPs you're using. Their types clearly show that they are built for a particular variant of fiber Ethernet, namely, 100FX. As opposed to metallic Ethernet where a NIC can speak all standardized speeds from 10Mbps up to its maximum supported speed (e.g. a GigE NIC speaks 10, 100, and 1000Mbps), an optical Ethernet module usually support only the specific transmission speed for which it was constructed, and no other. The reason is fairly simple: different variants of optical Ethernet usually use different wavelengths, necessitating different hardware in SFPs.

So the lack of the speed command is natural.

Best regards,
Peter

vinalsha Wed, 09/03/2014 - 05:41
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Hi Peter,

You are right.

I am using SFP like: QSFP40G-4SFP10G

With this SFP I have the option to configure different speeds and speed auto.

switch(config-if)# speed ?
  10     10Mb/s
  100    100Mb/s
  1000   1Gb/s
  10000  10Gb/s
  40000  40Gb/s
  auto   Auto negotiate speed

Depending on type of SFP, we can or we cannot configure "speed auto".

 

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