Networking »

Auto Negotiation

If locking the speed or duplex of a network card or switch, make sure you lock both ends of the link (i.e. cable). Locking the speed and/or duplex does not only lock it, but also at the same time stops the negotiation. This way, the other end have no means knowing what speed/duplex used. This may cause the different peers (e.g. PC and switch) running different duplex. It will seem to work until some load is applied, and then the performance will drop catastrophic due to excessive and uncontrolled collisions on the cable.

Autonegotiation (NWay) works quite well with gigabit equipment, but older network cards and switches do often have problems with it due to "creative" interpretations of the specifications.

Lock speed and duplex at either both ends, e.g. the PC and the switch, or at none at all. Note that auto crossover cable detection (mdi/mdx mode) may also be disabled and needs to be explicit specified.

References

• http://en.wikipedia.org/wiki/Autonegotiation
• http://en.wikipedia.org/wiki/Duplex_mismatch
• ethtool manual page
• mii-tool manual page

/By Mikael Q Kuisma