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
Best Answer
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.