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We have a network with a few switches. One of these is a 3com 4500, where there are many computers connected, a custom linux firewall and a wireless bridge. The port where the wireless bridge and the firewall are connected are in a separate VLAN, let's call it VLAN4. Port is in access mode, untagged. Everything else is in VLAN1, untagged.

When we disconnect the wireless bridge from the switch, connectivity is lost for a few seconds between random computers connected to the switch in VLAN1, then everything goes back to normal. During that period, our Nagios monitor displays many alerts regarding "DUPLICATE PACKETS FOUND".

We have isolated the firewall completely and it happens anyway, so it isn't the firewall.

Any ideas on why this happens?

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Is Spanning Tree enabled on the switch? And do you have any other information about those duplicate packets that are being reported, such as what device they were addressed to? – Shane Madden May 25 '12 at 16:48
Use WireShark on one of the affected computers and capture some traffic while recreating the problem. Perhaps a nature of duplicated traffic will be evident from the trace. If VLANs are working correctly, the broadcast or any other network traffic in the Wifi AP VLAN should not affect other VLANs. – Jo┼że Guna Jun 15 '12 at 14:49

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