Cisco 8851 auto negotiates at 100mb on 3850 & 3750 switches.

[Steve W]

We have Cisco 8851 phones that are auto negotiating at 100mb connection instead of 1000.  This is happening on both 3750X and 3850 switches (both have gig ports).  I did a shut, no shut on a few of the ports and some of the phones did register at 1000mb.  However several of the phones are staying at 100.  What could be causing this behavior?

[MHM Cisco World]

if the Phone not support autonego then the  SW will select 100mb by default 
so solution is 
disable autonrgo and then hardcore Speed 

[Leo Laohoo]

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@Steve W wrote:
However several of the phones are staying at 100.

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Check the config of the switch ports.

Check the length of the cable run.  

NOTE:  Do not make any attempts to perform a TDR using the 3850.

[Jonathan Schulenberg]

Along the lines of what Leo said: old/bad cabling or terminations would be my leading suspect. Does this happen when phones are directly connected to the switch new/known good patch cables?