Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
Community Member

Port Flapping

Hello All -

I have a couple 3550XL's, one  running 12.2(25)SEB4, the other 12.2(44)SE6.   The ports are trunked and each has a Motorola Canopy AP connected with 900Mhz Subscriber Modules providing wifi using Cisco Meraki devices.  I have hard coded each ports duplex and speed to full/100 and swapped cables on a couple of the ports, yet they are continually flapping with the %LINEPROTO-5-UPDOWN and %LINK-3-UPDOWN errors in the logs.

This happens all throughout the day, every day.  What could cause the flapping if I've hard coded each end and verified the cables are good?  Attached is the running config for the ports in question.  Let me know if there's any other info needed.  Thanks!

Hall of Fame Super Gold

Post the outputs to the

Post the outputs to the command "sh interface <BLAH>" of all the flapping interfaces.

Community Member

Hi Leo,I've attached the sh

Hi Leo,

I've attached the sh int for each flapping port.


Thanks for the reply!

Hall of Fame Super Gold

FastEthernet0/44 is up, line

FastEthernet0/44 is up, line protocol is up (connected)
     23446 input errors, 23446 CRC, 0 frame, 0 overrun, 0 ignored

Can you post the output to the command "sh controll e F0/44"?

Community Member

Thanks so much for your help.

Thanks so much for your help.  See below.  


Transmit FastEthernet0/44          Receive
1440836103 Bytes               2926725407 Bytes
  18585176 Unicast frames        12086477 Unicast frames
   4229831 Multicast frames         30300 Multicast frames
   5376566 Broadcast frames        132542 Broadcast frames
       137 Discarded frames          1276 No dest, unicast
         0 Too old frames               5 No dest, multicast
         0 Deferred frames            337 No dest, broadcast
         0  1 collision frames
         0  2 collision frames      24067 FCS errors
         0  3 collision frames          0 Oversize frames
         0  4 collision frames          0 Undersize frames
         0  5 collision frames     197845 Collision fragments
         0  6 collision frames
         0  7 collision frames    1126737 Minimum size frames
         0  8 collision frames    7486954 65 to 127 byte frames
         0  9 collision frames    1839745 128 to 255 byte frames
         0 10 collision frames     180254 256 to 511 byte frames
         0 11 collision frames     220001 512 to 1023 byte frames
         0 12 collision frames    1268893 1024 to 1518 byte frames
         0 13 collision frames
         0 14 collision frames          0 Flooded frames
         0 15 collision frames          0 Overrun frames
         0 Excessive collisions         9 VLAN filtered frames
         0 Late collisions              0 Source routed frames
         0 Good (1 coll) frames         0 Valid oversize frames
         0 Good(>1 coll) frames         0 Pause frames
         0 Pause frames                31 Symbol error frames
         0 VLAN discard frames          0 Invalid frames, too large
         0 Excess defer frames          0 Valid frames, too large
         0 Too large frames        197941 Invalid frames, too small
   6010850 64 byte frames          152355 Valid frames, too small
   4564201 127 byte frames
   1977112 255 byte frames
    943661 511 byte frames
    300834 1023 byte frames
  14394915 1518 byte frames

Hall of Fame Super Gold

Thanks Eric.   Please check

Thanks Eric. 



Please check the entire cable length to whatever is connected to port 44.  You've got a cable problem.   The output has high amount of FCS and collision fragments which are normally attributed to a faulty cable.


Community Member

if you have only one Switch

if you have only one Switch in network then try to t-shoot Leo has explained, if you have more than one switch in network then pls check STP on VLAN.



Nafis Ashique

Community Member

Hi Nafis Thanks for the reply

Hi Nafis 

Thanks for the reply and the advice.  If the cable doesn't fix the issue I'll check that and report back.

Community Member

Hi,if issue have in STP then


if issue have in STP then nothing depend on cable. can you send send the output of sho spannin.




Nafis Ashique

Community Member

Hi Nafis -I did replace the

Hi Nafis -

I did replace the cable and alleviated the input/CRC errors, yet we're still seeing the flapping occur.  I've attached the sh span.  Is it possible that clients roaming from one AP to another on property could cause a false-positive loop?  If the switch doesn't see that a device left one AP and connected to another, would it see that as a loop?


Community Member

As i found in show span

As i found in show span output this switch is not root switch so go to the root switch and run the below mention command, after that send the show span output


conf t

spanning-tree vlan 1-20 root primary



Community Member

OK, thanks for the advice.  I

OK, thanks for the advice.  I'll replace the cable and report back.

CreatePlease to create content