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I know this one is a tricky one because the problem could potentially be caused by quite a few different things. However maybe you've come across something similar or have some hints or tips of how I may be successful in debugging the current setup.

Basically we are running a small/medium sized office network with the current setup:

  1. Load balancer (Draytek Vigor 3300)
  2. Inside the load balancer we have 2 Cisco managed switches
  3. The load balancer is connected to 3 routes which supply the connection to the internet.

The problem is that every now and then the network(LAN and External) either slows down considerably, or the LAN is fine but external traffic comes to a complete standstill. This doesn't even occur in set intervals, we sometimes go weeks without noticing any problems. Restarting the load balancer solves the issue until the next time it happens.

Now, I appreciate that this is near impossible to solve without actually looking at the setup however I thought that maybe someone had been in a similar situation before or has any ideas what the problem might be.

If you need any more information please let me know

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up vote 2 down vote accepted

More Data:
Intermittent problems are tough. The first thing you need is to isolate the problem -- and to do this you need data:

  • Logs from all the devices including traffic logs.
  • CPU, Network Throughput and Packet Discards, and Memory Usage for all devices.
  • A packet capture during normal traffic and then during the problem if you can manage to get one.

Once you have all this data, you need to try to correlate the events with something that is happening with this data you have collected.

Or, Just Guess:
The other route you could take is just to start replacing stuff or simplifying the situation. With this method, you take your best guess and replace it. You say restarting the load balancer seems to fix this problem and that it is sometimes the LAN and sometimes the WAN. This points to the load balancer so that would be a logical place to start. You can get a lot of mileage out of HAProxy run on a commodity box for load balancing (i.e. generic Linux server) -- So you might try that.

Also, you might check for updates or open a support ticket with the load balancer manufacturer.

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We are having problems with a Vigor too, we had this problem a while ago, and was getting connection refused errors all the time. What FW are you running? the Lastest seemed to help.

On a different matter do you policy load balancing on your Vigor?

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We are running the latest firmware (Vigor3300 2.6.3 (EN)) when we experienced this problem. Yes, we are load balancing our 3 internet lines, however we used to run it with auto load balancing however we have now switched to static weighting as the auto load didn't seem to be very proactive in terms of trying to spread the load. – luxerama May 31 '11 at 8:40

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