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Why does my switch keep dropping connection to my firewall?

I have a Dell Sonicwall that becomes unreachable multiple times a day. The problem only lasts for about thirty seconds on average. There are three interconnected netvanta 1531's after the sonicwall and the "middle" switch is connected to it. About five times a day or better the SonicWall becomes unreachable from the network. It is reachable from the WAN side so the SonicWall doesn't shut down completely. There is nothing in the sonicwall logs to indicate any problems and I have already replaced the sonicwall unit completely so Ive taken bad ports out as a factor.

I have tried both a port redundancy to the switches and a link aggregation.  Adtran customer support told me to turn on the BPDU filter on those particular ports. The BPDU filter seemed to help a little bit but I still have the issue. I have tried everything that i possibly can, but since I'm just a dumb farmer maybe you guys can help me.

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Re: Why does my switch keep dropping connection to my firewall?

Hi Nick,

This does sound like a spanning-tree topology change issue that will clear in about that much time.

To see if you are having spanning-tree updates, use the command -

# deb span top

This will show a message if the topology changes causing all ports to be stopped ( blocked ) for that short few seconds.

#deb span bpdu    -   will show all spanning-tree packets

For more information -

Configuring Spanning Tree in AOS