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How to Troubleshoot I/O Port Flapping

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TitleHow to Troubleshoot I/O Port Flapping
Objective
Provide some causes of port flapping and steps to help resolve them
Environment
EXOS all
Procedure
Port flaps can occur for a variety of reasons, below describes some of them.
 
If a device is seeing intermittent connectivity issues between devices, one device’s log output may look like this:
Switch.1 # show log
06/16/2016 15:52:59.49 <Info:vlan.msgs.portLinkStateUp> Slot-1: Port 2:1 link UP at speed 1 Gbps and full-duplex 
06/16/2016 15:52:56.53 <Info:vlan.msgs.portLinkStateDown> Slot-1: Port 2:1 link down 
06/16/2016 15:42:51.72 <Info:vlan.msgs.portLinkStateUp> Slot-1: Port 2:1 link UP at speed 10 Mbps and full-duplex 
06/16/2016 15:42:49.20 <Info:vlan.msgs.portLinkStateDown> Slot-1: Port 2:1 link down
Port 2:1 is changing negotiated port speeds (from 10M to 1G) as well as going down and back up.
 
The following article has suggestions on what to check for this type of behavior, possibly due to auto-polarity not being enabled:

User complains about network connectivity due to port flapping, does Extreme support MDIX or Auto-MDIX feature on Extreme switches?

Additional things to check and commands to run when a port flap is experienced:
  • Where any recent changes made to the switch or network?
    • If yes, confirm the changes made and see if the changes may have caused the port flapping.
  • What type of device is the port connected to?
    • Some end devices go into a low power state which may temporarily shut down the I/O port and bring it back up when the device wakes up.  If the end device goes into a low power state occasionally, this behavior may be expected.
  • What time of day do the ports go down/up?
    • If there is a pattern to the times that the ports go down and up again, the end devices may be performing an activity during that time causing the ports to go down and back up.  The end devices can be reviewed to determine if the activity is expected or not such as powering up a desktop or logging off the network.
  • Get Show port txerrors output
Switch.3 # show port txerrors 
Port Tx Error Monitor
Port      Link      Tx          Tx          Tx          Tx       Tx       Tx
          State     Coll        Late coll   Deferred    Errors   Lost     Parity
================================================================================
1         A         10245        0           0         0         0         0
2         R         0            0           0         0         0         0
3         R         0            0           0         0         0         0
4         R         0            0           0         0         0         0
5         A         0            0           0         0         0         0
6         R         0            0           0         0         0         0
7         R         0            0           0         0         0         0
8         R         0            0           0         0         0         0
================================================================================
          > indicates Port Display Name truncated past 8 characters
          Link State: A-Active, R-Ready, NP-Port Not Present L-Loopback
If the Tx Collisions are seen incrementing, check port duplex setting with the command show port configuration.  Look for the Duplex Actual column.  If the value is half then collisions may occur causing connectivity issues.  Verify the duplex settings are set to auto on both sides of the link or hard code them to Full.
Switch.8 # show port configuration no-refresh 
Port Configuration
Port     Virtual    Port  Link  Auto   Speed      Duplex   Flow  Load   Media
         router     State State Neg  Cfg Actual Cfg Actual Cntrl Master Pri Red
================================================================================
1        VR-Default E       A    ON  AUTO    10 AUTO HALF   NONE        UTP         
2        VR-Default E       R    ON  AUTO       AUTO                    UTP          
3        VR-Default E       R    ON  AUTO       AUTO                    UTP          
4        VR-Default E       R    ON  AUTO       AUTO                    UTP          
5        VR-Default E       A    ON  AUTO  1000 AUTO FULL SY/ASY        UTP          
6        VR-Default E       R    ON  AUTO       AUTO                    UTP          
7        VR-Default E       R    ON  AUTO       AUTO                    UTP          
8        VR-Default E       R    ON  AUTO       AUTO                    UTP          
================================================================================
  • ​Get Show port rxerrors output
    • If the Rx CRC errors are seen incrementing, Check the port cable to determine if it may be faulty, swapping the cable with another one if possible.  If the cable is fiber cleaning the ends of the cable may be an option.
    • Since the packets with errors are coming into the switch, check the device on the other side.  It is possible that there could be a bad optic (if the cable is fiber), a device NIC driver needs updating or physical I/O port issue.  Replacing the cable or optic for a known good one can be done.  Changing the I/O port to another one may also help.    
Switch.4 # show port rxerrors 
Port Rx Error Monitor
Port      Link     Rx      Rx      Rx        Rx      Rx         Rx         Rx   
          State    Crc    Over    Under     Frag    Jabber      Align      Lost 
================================================================================
1         A        25      0        0        0        0          0          0
2         R        0       0        0        0        0          0          0
3         R        0       0        0        0        0          0          0
4         R        0       0        0        0        0          0          0
5         A        0       0        0        0        0          0          0
6         R        0       0        0        0        0          0          0
7         R        0       0        0        0        0          0          0
8         R        0       0        0        0        0          0          0
================================================================================
          Link State: A-Active, R-Ready, NP-Port Not Present L-Loopback
  • Get Show port info detail output​
Switch.5 # show port 1 info detail 
Port:   1
        Virtual-router: None
        Type:           UTP 
        Random Early drop:      Unsupported
        Admin state:    Enabled with  auto-speed sensing  auto-duplex
        Link State:     Active, 1Gbps, full-duplex
        Link Ups:       1        Last: Thu Aug 31 12:56:45 2017 
        Link Downs:     0        Last: -- 
Check the Link Ups and Link Downs output and see how many times the ports flapped and when the last one occurred. This can help confirm the amount of times the port has flapped since the last time the port counters were cleared.  This is also useful to determine if the port flap is occurring a lot or just a few times.
  • Get  Run diagnostics extended output. Along with testing the entire switch, it will test the physical I/O ports for problems:
 
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