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Link Speed Prbl.

I have 5600 series core switch and configure it with 3 vlans with 100 mbps/full duple, when i connect the server with auto obtain then its shows 100/half and unfortunately the traffic between network is very slow. please advice???                  


Link Speed Prbl.

When you have one side running auto and the 100/full what happens is:

  • Speed of the link is detected via Fast Link Pulses (FLP)
  • Duplex can not be detected so half duplex mode is assumed

Like you have seen this makes one end of the link end up in 100/half. I would recommend that you use auto negotiation on both sides as it has some error detection mechanisms that you don't have if you hardcode the links. If you set it to 100/full it will work as well.

Your speed is poor because you are in half duplex so the link will surely have collisions. When in half duplex you can only send or receive at once, not both.

Daniel Dib
CCIE #37149

Please rate helpful posts.

Daniel Dib CCIE #37149 Please rate helpful posts.

Link Speed Prbl.

   Easy to remember both sides must match , if you hardcode one side you "MUST" hardcode the other.  If set one side as auto then the other "must' be auto also .

Super Bronze

Re: Link Speed Prbl.


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Likely the auto/auto side is running in half duplex. 

Auto duplex will fall back to half duplex if it cannot negotiate full duplex with the other side.

With the hard coded side running in full duplex, many frames will be "corrupted" between the two interfaces.  Throughput is usually very poor in these situations, much, much worse than even if both sides were running in half duplex.  (Just to clarify, what Daniel wrote about half duplex mode, it's not so much half duplex that's degrading your performance, it's rather the two sides are using different duplex rules for transferring frames.)

You should configure both sides alike.  Today, most interfaces should be configured auto/auto.  (To clarify what Glen wrote, two sides don't always have to be configured alike to work correctly, e.g. 100/half and auto/auto, but they should be configured alike.)

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