Autonegotiating speed and duplex

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Nov 24th, 2008
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Hi All,


Cisco switches have the ability to autonegotiate the speed and duplex settings. Does this ability depend on CDP? Do we need to enable CDP to have this autonegotiation functionality running?


Cheers,


Sharjeel

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onurcoskun Mon, 11/24/2008 - 23:21
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By default cisco switch ports use Ethernet autonegotiation to determine duplex and speed settings.

Cisco switches detect the speed setting by using the Fast Link Pulses which is part of the autonegotiation process. However if the speed is manually entered on the other end of the link, the speed is detected based on the incoming electrical signal. Well, that means you do not need cdp for speed detection.


For duplex setting, both ends of the link must be configured to auto-negotiate. If one end is manually set to full/hall duplex, the other end cannot negotiate the duplex speed and assume the default setting depending on the ethernet connection type. (Half-duplex for 10/100mbps, full-duplex for 1000mbps)


This ability does not depend on CDP. But to detect any mismatch between two cisco devices, you need cdp enabled.


http://www.cisco.com/en/US/docs/ios/12_1/configfun/configuration/guide/fcd301c.html


http://www.cisco.com/en/US/tech/tk389/tk214/technologies_tech_note09186a0080094781.shtml


Hope this helped.

Joseph W. Doherty Tue, 11/25/2008 - 04:08
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If I remember correctly, autonegotiate is a hardware standard feature of (copper) FastEthernet and GigEthernet. It doesn't require CDP. For original Ethernet, autonegotiate may not be found on older equipment and might not function as desired, if available, on original Ethernet.


PS:

CDP can "flag" duplex mismatch, although with switches and devices connected to them, the error can be invalid between different devices connected to the switch.

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