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The switch is a Netgear GSM7248, we have three and all are flashing in unison. Any ideas?

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Can you run a network analyzer (wireshark, or something simmilar) on one of the interfaces, and see if there is alot of broadcast traffic? There could be a loop somewhere. – mulaz Dec 11 '12 at 11:06

There is probably an STP collision across the stack.

Log into the stack and re-start the STP loop discovery process.

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Check the CAM tables and see if they are full of 1-second old entries, that entirely consist of only one or a few MACs. – mfinni Dec 11 '12 at 16:41

Adaptr already mentioned STP issues.

Other possibility: Somebody is flooding the LAN with broadcast/multicast traffic.

As Mulaz suggested a Wireshark trace would be very helpfull.

You will see broadcast originating from the mac-address of the offender, or you will see a lot of spanningtree/BPDU packets come by.
Possibly both. One may lead to the other.

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Try one of the two answers above....or some of mine. everything i would try is listed here:

  • check the (R/M)STP Settings of the Switch
  • replace the Switch
  • decouple every Client connected to the Switch to see if blinking stops (looks like a packet-flooding)
  • try wireshark and check whats going on (maybe a management-port on the switch is helpful to see all packages)

Can you show us the network-topology?

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