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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
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    – Ron Maupin ♦
    Commented Aug 16, 2017 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 ♦
    Commented Jun 14, 2017 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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