Cisco switches blinking extremely fast

Posted on 2008-11-07
Last Modified: 2012-05-05
we installed new switches around 4 months ago.  we have several locations all conected by fiber.  before if we got a loopback the switches would blink super fast and eventually quit responding at the remote location(cisco 2950s).  now we have upgraded and have been running fine until 3 days ago, the network has slowed down but still works fine.  all the switches seem to be blinking insanely fast like if we had a loopback in the past.  not for sure what cisco commands are what to try to narrow it down. is there something to look for or try?  any help would be great
Question by:wavetech
    LVL 4

    Expert Comment

    do a sh int status on the switches and look for extreme high use of ports.
    blink super fast is relative. are they all blinking at the same time or port by port
    LVL 79

    Expert Comment

    Check "show proc cpu hist" and see if the CPU is spiking.
    Sounds like Spanning tree gone crazy.
    Try enabling multicast storm control on uplink ports of one switch and see if it settles down, then work your way back from there, enabling storm control on each uplink. Then you can check the storm control stats to try to pinpoint the source.

    LVL 3

    Expert Comment

    You can try a few things.

    Sh spanning-tree root will show you if something "out there" on the network has taken over spanning-tree functions from the root

    Sh cdp nei (detail) will let you see if you have a loop on a switch.

    Spanning-tree loopguard could be worth putting onto your bas config to allow the switches to err-disable any looped ports.

    Author Comment

    the lights are blinking fastly all at the same time.  sorry i am a cisco begginer.  what am i looking for when i do the spanning-tree root.  i assume when i do the sh cdp nei it will show the switch i am on if it has a loopback. i know im over my head but trying to learn. thanks again for the help.
    LVL 3

    Accepted Solution

    It should look something like this.

    sh spanning-tree root
                                            Root    Hello Max Fwd
    Vlan                   Root ID          Cost    Time  Age Dly  Root Port
    ---------------- -------------------- --------- ----- --- ---  ------------
    VLAN0001             0 0007.yyyy.xxxx         0    2   20  15
    VLAN0003         32768 0006.aaaa.bbbb     3012    2   20  15  Gi7/14
    VLAN0005             0 0007.yyyyxxxx         0    2   20  15
    VLAN0006             0 0007.yyyy.xxxx         0    2   20  15

    In the example here VLAN1, 5 and 6 are all on the switch I am connected to for their spanning-tree root.

    You can also see that VLAN 3's root has a different MAC address and is "known" down port Gi7/14.

    A Sh cdp nei detail gigabit 7/14 detail ( gigabit 7/14 because my port above is gi7/14 so substitute your own ) will bring you to the device that is the root of that SP for that VLAN.  If it's a link off a link you may have to do this once or twice more to get there - you will know when you are there as the port will be blank.

    when on that switch do a sh cdp nei which will return something like... ( Anonymised by me)

    Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
    Router           Gig 0/1           154          R S I     WS-C6509  Gig 3/1
    roof             Fas 0/40          125            T       AIR-BR350 fec0
    switch           Gig 0/2           125           S I      WS-C3560- Gig 0/2
    Switch2          Fas 0/11          152           T S      WS-C3548- Fas 0/4

    Take note of your switch name ( the bit before the # ) and if it appears under DeviceID then you have a loop.  Local Intrfce and PortID will tell you the 2 ports that are looped.

    Author Closing Comment

    thank you. i found the loopback.

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