Ethernet Duplex Issues

The speed can be negotiated by using Fast Link Pulses (FLPs), but it can also be derived by the received electrical signal.

The duplex setting can only be discovered by negotiation (ie sending / receive FLPs). If none are sent then the interface defaults to half duplex (HDX).

If you have a link that is half duplex at one end and full duplex at the other then:

The HDX end will report late collisions as its transmission are interrupted.
The FDX end will report Frame Check Sequence (FCS) errors as the frames are stopped short.
The FDX end will show collisions where there should be none.

  • Runts – Runts are frames smaller than 64 bytes
  • CRC errors – The frame’s cyclic redundancy checksum value does not match the one calculated by the switch or router
  • Frames – Frame errors have a CRC error and contain a noninteger number of octets.
  • Alignment – Alignment errors have a CRC error and an odd number of octets
  •  Collisions—Look for collisions on a full-duplex interface, or excessive collisions on a half-
    duplex interface.
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