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Newer Cisco IP Phones may fail to autonegotiate a link to 100 / Full on C9300-48T/P/U and C9300-24T/P/U Legacy IP phones (such as C7900 series phones) will still autonegotiate. This occurs specifically when using ethernet twisted-pair cabling where only pairs A and B are connected. This means the cabling only has Pins 1,2,3 and 6 terminated.
Devices are typically newer mode IP phones (Cisco C8800 and newer series phones) or CP-DX series phones. The cabling used *must* be half-way paired such that it only supports 100BASE-TX. If the cabling is normal ethernet cabling, then this issue will not be encountered.
Configuring "speed 100" on interfaces with this problem will disable autonegotiation and allow the port to come up into the 100 / Full state. Swapping the cabling for regular fully-paired ethernet cabling will also correct this issue.
To check and see if the cabling in use can cause this problem, use the TDR tester available on the switch. To start the test, the following command in EXEC mode may be issued: C9400# test cable-diagnostics tdr interface G2/0/2 This test requires a few seconds to run, after which point you may display the results like so: C9400#show cable-diagnostics tdr interface G2/0/2 TDR test last run on: January 23 02:11:22 Interface Speed Local pair Pair length Remote pair Pair status --------- ----- ---------- ------------------ ----------- -------------------- Gi2/0/2 1000M Pair A 0 +/- 10 meters N/A Normal Pair B 162 +/- 10 meters N/A Normal Pair C 0 +/- 5 meters N/A Open Pair D 0 +/- 5 meters N/A Open If Pairs C and D list as "OPEN" status, that link is vulnerable to this issue.
The Cisco PSIRT has evaluated this issue and does not meet the criteria for PSIRT ownership or involvement. This issue will be addressed via normal resolution channels. If you believe that there is new information that would cause a change in the severity of this issue, please contact psirt@cisco.com for another evaluation. Additional information on Cisco's security vulnerability policy can be found at the following URL: https://tools.cisco.com/security/center/resources/security_vulnerability_policy.html