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network blips every couple minutes

Posted on 2016-09-06
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Last Modified: 2016-09-14
We have a bunch of 24 port switches - each plugged into a port on our firewall with it's own subnet.

All of the clients attached to a particular one of the switches would lose most connectivity every couple minutes for a minute or so.

so for instance
192.168.31.15 would be pinging
192.168.31.1 (it's own gateway)
192.168.1.1 (gateway of servers IP)
192.168.1.7 (main server)

1.1 and 1.7 would fail, but somehow 31.1 would still reply.... BUT firewall company says the packet never got to them because sniffer saw nothing come in during that time.....

Anyway - so we rebooted switch - issue persisted - so we moved all the cables from that switch to another switch on another subnet... things are working fine so far...

strange thing is we can plug in a laptop to the original "Bad" switch and now pings run fine on it....

Totally confused - any idea what the issue could have been?
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Question by:Xetroximyn
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ArneLovius earned 500 total points
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excepting changes to the local routing table, if the gateway address is on the firewall, and pings to the gateway were okay and pings to addresses beyond the gateway were not, then the problem is not the local switch but either the firewall or beyond the firewall.
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by:Xetroximyn
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firewall tech was doing a sniffer - claims that nothing was coming into firewall while at same time my guy onsite says he was getting replies.... I wonder if something could have somehow been static set to same ip as gateway and fighting it?  

Anyway, Things are still all good as it stands.... just so weird.... wonder if it could have been overheating?  Traffic flows, causes heat, causes problems, traffic stops, it cools down, traffic flows again.  But that doesnt explain how we got ping replies from 30.1 when fortigate sniffer saw no such traffic.
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by:ArneLovius
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it would be "unusual" for a L2 switch to overheat because of traffic flow...

something else taking the default gateway address could cause the problem you saw, the way to check if it happens again is to check the ARP cache with "arp -a" and see if the MAC address matches the expected MAC address
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by:Xetroximyn
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Thanks - should have checked arp when we were troubleshooting!!   Oh well - it's working now, so I am leaving it.

Question... would the fact it's a server room with not great airflow, a completely mess of cables, and is about 76 degrees because the A/C unit is not quite as powerful as it should be change your mind at all about the overheating probability?   (Also the switch is probably 10 years old.... and likely never air dusted.... so dust may have collected in decent quantities by now.)
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by:ArneLovius
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heat "may" be an issue, do the switches have a management interface ?

if the switches are ten years old, any initial warranty is long gone, so you could open them and give them a blast from a can of compressed air.
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