Cisco 1710

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Aug 21st, 2007
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Maybe it's a silly question.. I have a modem which is connecting with cisco1710 router on its E0. This E0 is setup as half-duplex and I experienced 60000 collisions in last one month and half. I called ISP and they told me their modem can run on both half or full duplex. Can I set the router to full duplex? Will the modem automatically pick up new mode? Thanks in advance!

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royalblues Tue, 08/21/2007 - 13:37
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yes you should configure your interface to full duplex.

i do not know whether the modem can be hardcoded to full duplex. Mostly they are auto-sensing. It would be worthwhile to set your port to auto and see what it negotiates with the modem



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jwdoherty Tue, 08/21/2007 - 16:00
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Unless you can configure your modem to full duplex, do not set your router to full duplex. Either leave the router's port at half or set to auto.

What you don't want is one side half and the other side full. Traffic will usually pass, but very, very slowly.

If auto fails, which it generally does with the other side being full, it reverts to half.

If the modem does support auto, it, and your router port, should go to full. If they don't both go to full, they both should revert to half.


Although full duplex avoids collisions, they are part and parcel with half duplex Ethernet. For just two devices on the wire, unless both are transmitting much, the collisions don't often have a huge impact since Ethernet will see them and retransmit the frame.


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