Duplex Modes and Auto-Negotiation

A point-to-point Ethernet segment (typically between a switch and an end-node, or between two directly connected end-nodes) can operate in one of two duplex modes: half duplex means that only one station can send at a time, and full duplex means that both stations can send at the same time. Of course full-duplex mode is preferable for performance reasons if both stations support it.

Duplex Mismatch

Duplex mismatch describes the situation where one station on a point-to-point Ethernet link (typically between a switch and a host or router) uses full-duplex mode, and the other uses half-duplex mode. A link with duplex mismatch will seem to work fine as long as there is little traffic. But when there is traffic in both directions, it will experience packet loss and severely decreased performance. Note that the performance in the duplex mismatch case will be much worse than when both stations operate in half-duplex mode.

Work in the Internet2 "End-to-End Performance Initiative" suggests that duplex mismatch is one of the most common causes of bad bulk throughput. Rich Carlson's NDT (Network Diagnostic Tester) uses heuristics to try to determine whether the path to a remote host suffers from duplex mismatch.

Duplex Auto-Negotiation

In early versions of Ethernet, only half-duplex mode existed, mostly because point-to-point Ethernet segments weren't all that common - typically an Ethernet would be shared by many stations, with the CSMA/CD (Collision Sense Multiple Access/Collision Detection) protocol used to arbitrate the sending channel.

When "Fast Ethernet" (100 Mb/s Ethernet) over twisted pair cable (100BaseT) was introduced, an auto-negotiation procedure was added to allow two stations and the ends of an Ethernet cable to agree on the duplex mode (and also to detect whether the stations support 100 Mb/s at all - otherwise communication would fall back to traditional 10 Mb/s Ethernet). Gigabit Ethernet over twisted pair (1000BaseTX) had speed, duplex, and even "crossed-cable" (MDX) autonegotiation from the start.

Why people turn off auto-negotiation

Unfortunately, some early products supporting Fast Ethernet didn't include the auto-negotiation mechanism, and those that did sometimes failed to interoperate with each other. So many knowledgeable people recommended to avoid the use of duplex-autonegotiation, because it introduced more problems than it solved. The common recommendation was thus to manually configure the desired duplex mode - typically full duplex by hand.

Problems with turning auto-negotiation off

There are two main problems with turning off auto-negotiation

  1. You have to remember to configure both ends consistently. Even when the initial configuration is consistent on both ends, it often turns into an inconsistent one as devices and connectinos are moved around.
  2. Hardcoding one side to full duplex when the other does autoconfiguration causes duplex mismatch. In situations where one side must use auto-negotiation (maybe because it is a non-manageable switch), it is never right to manually configure full-duplex mode on the other. This is because the auto-negotiation mechanism requires that, when the other side doesn't perform auto-negotiation, the local side must set itself to half-duplex mode.

Both situations result in duplex mismatches, with the associated performance issues.

Recommendation: Leave auto-negotiation on

In the light of these problems with hard-coded duplex modes, it is generally preferable to rely on auto-negotiation of duplex mode. Recent equipment handles auto-negotiation in a reliable and interoperable way, with very few exceptions.

References

-- SimonLeinen - 12 Jun 2005 - 4 Sep 2006

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Topic revision: r4 - 2006-09-04 - SimonLeinen
 
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