ZyXEL router error message

My (old) ZyWall 2 router/firewall is giving error message that I haven't been able to find the source for. It is configured as a plain router (static wan IP), DHCP server on the LAN side, and what is called Full Feature NAT. Everything seems to work well, but many times every day there are error messages (alerts) saying:

No. Time                  Source IP              Destination IP         Note
  1|08/17/2014 11:37:17  |                      |                      |
    Discard packet, the vlan tag is undefined. vlanTag=7
End of Alert

I have no equipment on the LAN side using VLAN. WAN side is having an official IP address (part of my xxx.xxx.xxx.xxx/25 network). Since the alert doesn't indicate any source or destination address, it seems difficult to debug.

Is there a way to find what is causing these alerts?
Olaf BerliAsked:
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Chris GralikeSpecialistCommented:
Hi Omberli,

If your able, you might want to sniff the traffic using wireshark.

If there is any 'junk' on the line, you should be able to capture that.
You could be dealing with malformed packets. Additional packet headers like media address might help you sort this problem.

If all is working as planned, than the alternative is to accept the error.

you can also run the trace on the zywall itself ( http://www.zyxeltech.de/snotezw5_362/trbst/trace.htm )

but you probably should not care that much. you may have a misconfigured switch, or more likely your provider has. you should be able to determine which by first finding out which interface receives the packet.

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Olaf BerliAuthor Commented:
Tried Wireshark, but it didn't find anything (connected it directly to a router port to avoid any switches to make problems). Also tried the console commands on the ZyWALL, but none of the command worked. All of them gave an ERROR as response.

Seems like I end up just ignoring these alerts. Thanks anyway!
wireshark on the zyxel would only work assuming you set up the port as a copy of another port's traffic which i'm not even sure is feasible on the router

if you want to run a trace on the fw, the zyxel kb shows a set of different and simpler commands. http://kb.zyxel.com/KB/searchArticle!viewDetail.action?articleOid=013652&lang=EN

... but it is very likely that you actually can ignore the error
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