Planet WDRT-731U User Manual page 46

 Speed/Duplex
This section allows you to config the router's WAN port speed/duplex settings.
You can select a WAN port speed/duplex mode that best suit your network environment from the drop-down list,
which includes auto, 10M half-duplex, 10M full-duplex, 100M half-duplex, 100M full-duplex, 1000M half-duplex
and 1000M full-duplex.
The WAN port speed/duplex mode must match that of its link partner to achieve
successful communication; otherwise, the WAN port may not function properly. So, if
you are not sure about the link partner's speed/duplex mode, please select "Auto"
Figure 5-2-14
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User Manual of WDRT-731U