Duplex mismatch

scmuser created the topic: Duplex mismatch

Symptom: With a duplex mismatch, the connection will work, just poorly. The link lights will become active on both the switch and network adapter. Network performance will suffer greatly, with throughput dropping to 100Kbps or lower.

sgadmin replied the topic: Re:Duplex mismatch

Cause: One side of the connection is operating in full duplex (transmit and receive at the same time) and the other device is operating in half duplex (transmit OR receive at one time). The full duplex side does not have to wait to transmit, it is configured to transmit whether it is receiving or not. The half duplex side must wait until it is not receiving in order to transmit. This means that the full duplex side has the potential to interrupt the half duplex side, causing the half duplex side to abort
transmission. If transmission is aborted, the frame will need to be retransmitted. This will dramatically reduce the bandwidth the half duplex side is able to make use of.
Resolution: In nearly all cases, a duplex mismatch is the result of forcing one side of the connection (usually the switch) to full duplex, while leaving the other side, the PC, to auto-negotiate the link. The myth is that auto-negotiation will determine the forced full setting and match this configuration. This however is not the case. The side forced to full duplex is no longer sending the appropriate signaling that auto-negotiation relies on in order to determine speed and duplex. The auto-negotiating side of the connection will be left to guess at the duplex of the link. When in doubt, auto-negotiation will always default to half duplex. This is how most duplex mismatch issues happen on the network. To resolve this, set all connections on the network to auto-negotiate – unless you have reason not to. In these isolated cases, such as inter-switch connections, be sure to statically set both sides to full duplex.

Rajesh Kumar
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