I was just learning about auto-negotiation and got some doubts.

As per the standard 802.3 clause 28, 10BASE-T ports transmits LIT or NLP pulses to make the link UP.


Also when 10/100/1G (Auto-neg enabled) port is connected to 10M FD (auto-neg disabled) port, FLP from Auto-neg port and NLP from Auto-neg disable port will be transmitted, result is 10M HD and 10M FD respectively.

  1. What will happen when 10/100/1G (Auto-neg enabled) port is connected to 100M FD (auto-neg disabled) port ?
  2. Does 100M port use the same LIT/NLP ?
  3. What signaling method does 100M port use when 2 FE ports (Autoneg disabled) are connected to each other ?

Thanks in advance for reading

2 Answers 2

  1. Usually the port comes up - due to lack of autonegotiation, the auto side will choose half duplex, resulting in a duplex mismatch.

  2. Fast Ethernet uses "fast link pulses", an expanded scheme.

  3. Autoneg disabled on both sides leaves only the carrier (pause symbols) to detect the link. FDX/HDX will be as set up, potentially mismatching.

  • So autoneg enabled port will transmits FLP s, and autoneg disabled port will transmit pause symbols. And when both ports autoneg is disabled, both transmits pause symbols. Can u just explain little more on these 'pause symbols' ? or is there any docs which i can read ? Thanks in advance
    – Anilal
    Commented Aug 9, 2017 at 14:50
  • You should check IEEE 802.3 itself: ieeexplore.ieee.org/browse/standards/get-program/page/… For FE, you need Section 2, Clauses 22, 28 and especially for non-autoneg links.
    – Zac67
    Commented Aug 9, 2017 at 15:08

Cisco has a document, Troubleshooting Cisco Catalyst Switches to NIC Compatibility Issues, that describes this, that includes a table that tells you what happens at various settings.

A duplex mismatch usually occurs when on side of a link is set to auto-negotiate, but the other side of the link is set to a fixed speed. When this is the case, the side set to auto-negotiate cannot negotiate. It will detect the fixed speed, and it will default to the duplex for the speed (half duplex for 10 or 100 Mbps).

A duplex mismatch will cause a lot of input errors, collisions, runt, giants, etc. Your throughput will be horrendous. Things will seem to work very slowly.

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