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1.
Verify port status and troubleshoot accordingly. Refer to the Port status shows not connected, faulty,
disabled, inactive, or errdisable section of this document.
2.
Capture the output of the show port <mod/port> , show mac <mod/port> , and show counters
<mod/port> commands.
These are common causes for data link errors on ports:
♦
speed/duplex misconfiguration
♦
network congestion
♦
NICs or drivers
Refer to Troubleshooting Cisco Catalyst Switches to NIC Compatibility Issues for more
information.
♦
cabling
♦
bad port
The show port <mod/port> command can show Late−Coll, Align−Err, FCS−Err, Xmit−Err, and
Rcv−Err errors. Refer to the the Show Port for CatOS and Show Interfaces for Cisco IOS section of
Troubleshooting Switch Port Problems for more informaiton on these errors and possible causes.
The show mac <mod/port> command shows the number of unicast, multicast, and broadcast frames
transmitted. Issue this command in order to verify if frames are received and transmitted.
In−Discards show frames that do not need to be switched. This is normal if the port was connected to
a hub and two devices exchanged data. Lrn−Discards indicate that Content Addressable Memory
(CAM) entries are being discarded. In−Lost counter displays the sum of all error packets received on
the port. The Out−Lost counter indicates egress port buffer overflows. Refer to the Show Mac for
CatOS and Show Interfaces Counters for Cisco IOS section of Troubleshooting Switch Port Problems
for more information on these errors and possible causes.
The show counters <mod/port> command is useful in particular for troubleshooting port problems.
For example, this counter results if you issue the command:
5 badTxCRC = 0
If badTxCRC were incrementing, this can be bad hardware corrupting packets. Capture the output of
the show counters <mod/port> command and open a case with the Cisco Technical Support.
3.
Issue the clear counters command in order to reset the output of the show port <mod/port>, show
mac <mod/port>, and show counters <mod/port> commands. View the command outputs several
times in order to see if errors are incrementing.
If you have not been able to track down any reason for intermittent connectivity loss on the switch in
the previous steps mentioned, capture the output of the show nvramenv 1 command, as well as the
other commands in the previous steps, and open a case with the Cisco Technical Support.
4.
Refer to these documents for more information on how to troubleshoot the other causes of port errors:
♦
Troubleshooting Cisco Catalyst Switches to NIC Compatibility Issues
♦
Configuring and Troubleshooting Ethernet 10/100Mb Half/Full Duplex Auto−Negotiation
Experiencing poor performance
Poor performance is often perceived to be a hardware problem, when in fact it can be attributed most often to
connectivity problems. See the Seeing errors on the ports section for troubleshooting steps.