Duplex Mismatch

Or why interface auto negotiation is a bad idea

Current Network Interface Cards (NIC) come in with the following four possible bandwidth configurations.
  • 10 Mbps ½ duplex, typically used with low speed hubs
  • 10 Mbps full-duplex, typically used with low speed switches
  • 100 Mbps ½ duplex, typically used with high speed hubs
  • 100 Mbps full-duplex, typically used with high speed switches
* Switches, unlike Hubs can be configured for both full-duplex or half-duplex. Hubs can only operate at half-duplex.
**Gigabit Ethernet is becoming more common, but not in the desktop environment yet.

These settings can be manually configured or left to the auto negotiation mechanisms. The auto negotiate mechanisms do not always work. A server manufactured by Sun Microsystems, connected to a Cisco switch a a common example of the problem.

The half duplex configuration allows a NIC to only send or receive data at any given instant, while full duplex allows a NIC to send and receive data simultaneously.

When a NIC is configured for full duplex mode, it does not check to see if there is any signal from another NIC on the wire before it starts to transmit. The NIC simply transmits traffic as it is assumed that both sides of the wire will be full-duplex. Therefore, on a switch port with excessive collisions, there is most likely have a duplex mismatch condition.

Any network segment with a collision rate greater then 1% of the number of transmitted packets is considered to have an excessive collision rate. Collisions can also aid in the trouble shooting of other network conditions. On a shared Ethernet segment, the occurrence of late collisions typically points to an excessive length of wire or network diameter. A late collision happens when the collision detection signal is received after the transmission of the first 60 bytes of the Ethernet frame. The time required for transmitting 60 bytes is approximately 50.2 microseconds. A segment that has a collision the will never have a wire-length which exceeds the Ethernet specifications based on this time limit.

Collisions are detected by the voltage on the wire exceeding the normal operational value of approximately 5 volts.

On a shared Ethernet, segment collisions are a normal operating condition of Ethernet, just watch for late collisions and excessive collisions.
Originally published Jan 20, 2004. Updated Feb 7, 2009


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