I checked this. It appears to be completely symmetrical at the switch level. 
For both the 1Gb and 100Mb ports, the delta I see for NTP packets is a very 
consistent 912ns on a 1Gb mirror port. This is exactly as expected:

  preamble: 56 bits
  SFD: 8 bits
  MAC dest: 48 bits
  MAC src: 48 bits
  Length: 16 bits
  IP/UDP/NTP: 608 bits
  FCS: 32 bits
  IPG: 96 bits

Total of 912 bits, which equates to 912ns on a 1Gb connection.

NB: The packet cannot be forwarded until it is received, so the preamble and 
IPG count when looking at packet to packet deltas when mirroring.

I’ll have to look at the driver/kernel next. It may just be an inherent offset 
due to software timestamping. I may hard code a correction for the hardware 
timestamps in chrony for testing.


> On Nov 21, 2016, at 10:50, Denny Page <dennyp...@me.com> wrote:
> I am going to set up a span port to capture the packets on both interfaces 
> which may give an indication.

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