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My company just changed it's old firewall (Zyxel ZyWALL 70) with a new one (Zyxel ZyWALL USG 300).

Everything seems to be working fine but sometimes, in a non-periodic span of time, connectivity seems to drop.
Web navigation stops, ftp uploads get interrupted and email checking fails.

EDIT: The interruption lasts 10-15 seconds.

Everything works if we plug the old firewall, so we are trying to understand what might cause the problem.
Connection is not dial-up with firewall but via ethernet with an external modem (unchanged from previous configuration).

Accordingly to log, WAN interface is never shut down.

How can we track down the issue?

Please post a comment to this question if you need any further information.

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This is not a cheap device. It should have some log files. Have you checked that out? – wolfgangsz Aug 27 '10 at 10:01
We've had randoms problems with an Zyxel USG300 dropping VPN connections (80+ VPNs). Zyxel support was no help although we gave them access to local and remote (Zyxel) configs, we finally end up replacing it (with a Juniper). The precedent Zywall 70 was performing ok. – Maxwell Aug 27 '10 at 10:06
Can you connect to the Zywall while the issue is ongoing? – Cian Aug 27 '10 at 20:23

You might check the MTU of the new unit against that of the old one.

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Thanks for your answer. Couldn't find MTU as a configurable firewall side parameter. I found it in the modem, but that is unchanged. – Alex Bagnolini Aug 27 '10 at 8:27
You might check with the manufacturer. During the disruptions, you might also check the amount and type of traffic flowing across the router. That may also lead to additional hints as to what may be taking place. – user48838 Aug 27 '10 at 22:31

Heat related failure?

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Thank you for your answer. The whole room is kept constantly at 20°C. The strange thing is that the interruption lasts only a bunch of seconds, and everything then returns to normality (just added this information in the question). – Alex Bagnolini Aug 27 '10 at 8:32

You could check the switch(es) sitting on the ports of the device for strange behaviour. If the new device causes some problems (a) on the subnet (check for excessive traffic one doesn't notice until something else breaks, e.g. ARP, broadcasts etc.) or (b) directly on the port of the switch to have a component fail (electrical problems, e.g. too much potential difference on the shielding a.k.a grounding problems, static buildup etc.).

Either side of the connection could reset its port (thus the short time), having the problem going away and reappearing after (a) some buffer runs full or (b) enough potential has built up again.

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