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If you check show int tranceiver command on Nexus Switch 10Gbit Ethernet port, the output shows the nominal bitrate is 12000 MBits/sec instead of 10000 MBits/sec. As I know 12 bytes Interframe Gap + 8 bytes preamble + 18 bytes frame header and FCS are all included in bandwidth 10000 MBits/sec of 10Gig Ethernet. So what is 2000 Mbit/sec overhead used for?

switch# show interface Ethernet1/5 transceiver
Ethernet1/5
sfp is present
name is CISCO-MOLEX INC
part number is 74752-9025
revision is A
serial number is MOC12302468
nominal bitrate is 12000 MBits/sec
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 16 '17 at 21:39
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The nominal bit rate is the symbol rate. Some Gigabit Ethernet standards uses 8b/10b encoding, which inflates the line rate from 1 Gbps to 1.25 Gbps.

For 10 Gb Ethernet, it normally uses 64b/66b encoding and shouldn't have such a high symbol rate, so I'd say there is an additional reason that inflates the line rate.

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    Although nearly all 10GE PHYs use 64b/66b encoding (10GBASE-xR), 10GBASE-LX4 (legacy fiber), -CX4 (twinax and some DAC), and -KX4 (backplanes/blades) use 8b/10b encoding. Since they also use FEC, actual line rate is 4* 3.125 Gbit/s. – Zac67 Jun 14 '17 at 18:09
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Transceivers are most often designated by the standard transmission speed on the medium (e.g. 1.25 Gbit/s for Gigabit Ethernet or 10.3125 Gbit/s for 10 Gigabit Ethernet), but sometimes they are labeled with their nominal Ethernet speed or a higher speed the manufacturer specifies. This is because it operates in a connectionless mode and nominally (due to possible packet loss) offers e.g. 1.25 Gbit/s.

Source: https://books.google.dk/books?id=ha0veu076xoC&pg=PA22&lpg=PA22&dq=nominal+Ethernet+speed+1.25+gbit&source=bl&ots=-MfGInXlrH&sig=k2SEfRfLd11mySyNxcR3TgpfI44&hl=da&sa=X&ved=0ahUKEwjCut3Rnr_UAhUSJ1AKHQlgALsQ6AEITzAF#v=onepage&q=nominal%20Ethernet%20speed%201.25%20gbit&f=false

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