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I have a Cisco 881w with an AP.

Sometimes, the wifi just drops its clients and the wifi network disappears. The wired network is still working fine.

Also the wifi network reappears after a couple of minutes.

How can I troubleshoot this issue please?

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I would start with looking at the router logs by using show log.

Like any cisco, if logging is enabled, you should see at least some %DOT11 messages like %DOT11-6-ASSOC, %DOT11-6-DISASSOC...

To enable logging with a 10kB buffer (default is 4096bytes):

router#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
router(config)#logging buffered 10000 

If nothing obvious appears from the logs, you could try to activate some debugs on the router:

router#debug dot11 ?
  Content-Engine     cisco content engine service module
  Dot11Radio         IEEE 802.11 WLAN
  Service-Engine     cisco application engine service module
  aaa                Authentication, Authorization, and Accounting
  arp-cache          ARP Cache
  events             IEEE 802.11 events
  forwarding         802.11 AP forwarding
  mgmt               802.11 Management
  network-map        Network Map
  packets            IEEE 802.11 packets
  syslog             Turn Off Dot11 syslog msgs
  virtual-interface  802.11 virtual interface

That should help to troubleshoot the issue.

Be sure also to disable logging console as logging on the serial console generates a cpu interrupt for any character (hence a high cpu usage), and prefer terminal monitor if you're debugging from a vty (telnet/ssh).

A syslog server would be quite useful also. It will store all the logs from the router, and they can then be processed (script of some kind, your eyes...) on the server.

router(config)#logging ?
  Hostname or A.B.C.D  IP address of the logging host

router(config)#logging <ip of syslog server>
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How can I see the log files afterwards ? – Jonathan Rioux May 24 '12 at 15:34
@JonathanRioux: see my edited answer. – petrus May 24 '12 at 21:36

At first blush I would look for sources of RF interference, which are quite common in the 2.4GHz band. Microwave ovens and cordless phones are often the culprits but there are plenty of other things that can cause problems.

I'd also suggest setting up a wireless sniffer on a laptop (or even a droid phone) and watching the general signal levels before, during and after a service interrupting event. There are quite a few out there to choose from based on your platform - various iterations of stumbler, setting up wireshark to run in monitor mode, etc.. These tools should let you get some sense of whether it's a general blast of RF causing your problems or something from a specific station and/or rogue AP.

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How can I see if the interferance is really the issue ? – Jonathan Rioux May 30 '12 at 20:11
Watch the spectrum analyzer. If it seems to go nuts while the issue is occurring then chances are that it's interference. If so, you might be able to get around it by manually setting your AP to the very top or bottom of the frequency range... – rnxrx May 31 '12 at 21:50
What if I dont own a CleanAir device? – Jonathan Rioux Jun 1 '12 at 4:08
I wasn't suggesting a CleanAir device, although that would certainly address the issue. There are a number of free or cheap tools out there (example - ) that can provide spectrum analysis to at least give you a clue as to what's going on. – rnxrx Jun 1 '12 at 4:24

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