I am beginner in networking and I have two questions, because I can't understand it well.

  1. I know how auto-negotiation works, but what happens if two connected devices have different speeds and auto-negotiation is disabled? What happens if only one device has auto-negotiation disabled?
  2. What happens if I connect 100 Mb/s ethernet to hub (switch) with/without auto-negotiation?
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    Aug 10, 2017 at 22:55

3 Answers 3


Two devices operating at different set speeds will not link. If only one side is auto, they may link, but it depends on the specific device(s) as some require the link-pulse information while others will cycle through speeds to get a link; and in the absence of link-pulse information, they will default to half-duplex. (Note: the non-negotiating side may have been set to full-duplex, but the other side will be half-duplex.)

In your second case, a hub is always half-duplex; as long as it supports 100Mbps, it will link. A switch can run in either full or half-duplex. Same as before, without the information from link-pulses, they will default to half-duplex. (unless configured otherwise.)


There are two different things to think about, each can be enabled or forced:

  • Speed Negotiation (Auto, 10mbps, 100mbps, 1gbps, etc..)
  • Duplex Negotiation (Auto, Full, Half)

Speed negotiation is normally pretty easy. As long as both ends support the same speed (you don't force the speed on each end to a different ones or there are speed limitation because of hardware - some SFPs that support 1Gbps only and not 100Mbps, etc.) you'll link-up.

Duplex negotiation (or lack of) will not prevent linking, but any mismatch will create extreme slowdowns. You can find those by checking stats: One end sees a lot of Collisions while the other has no collisions but lots of FCS Errors. Basically, the full duplex end sees incomplete frames sent by the half-duplex that stopped mid-transmission because of a collision. (I've seen 10Mbps

A Few more info:

  • For 10, 100mbps links, Duplex negotiation failure means the interface falls-back to HALF duplex
  • For Gigabit links, Duplex negotiation failure means it falls-back to FULL duplex
  • HUBS do not support Full Duplex, nor speed negotiation (all ends must be at the same speed and HALF duplex). Hubs are mainly "cross-over wires" for every ports.
  • Unmanaged switches will always negotiate Speed and duplex (and provide a buffer between links different speed)
  • Managed switches can have any components use negotiation or forced

Use a rule: if only one device has auto-negotiation disabled, then it won't work. The standard says auto-negotiation will work only if both sides are configured to use it so there'll be very small chance it'll work always and as intended if you set it up on one side only. So never trust such a config even if it appears to work at first glance.

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