Expand all | Collapse all

How to Begin Troubleshooting Slow Network Issues

Jump to Best Answer
  • 1.  How to Begin Troubleshooting Slow Network Issues

    Posted 05-16-2017 14:55



    Although I've been working in IT and with networking for 10 years, I'm still fairly new to the world of switching and routing.  Our network has grown to the point that it's about a mid size network with 200+ computers, devices, APs, switches, and so on.  We use EX2200s on most of our access layer switches and a Cisco SG300 for our core switch.  What we're running into is a situation where we are trying to run a backup on a group of computers and we think it's a network topology or some other related issue.  e.g. I have tested the backup times by connecting a computer directly to the backup server with no switches in between and the backup takes 2 hours.  If we run the backup from another computer across the network over copper and fiber and 4 intermediate switches (some fairly cheap ones as well as an EX2200), with approximately similar hardware it takes almost 5 hours.


    I'm not asking anyone to figure this out for us, but I just don't know where to start, and I'm not even sure of the right questions to ask.  Is this a VLAN issue?  Is it STP related?  Is it a topology thing?  We don't have any redundant connections that we know of that would cause loops.  Is there some way to trace the route of packets across the network so we can see how a packet is routing?  Also, our EX2200s are configured with some basic VLANs, and that's about it.  Our network is fairly plain vanilla and VLAN traffic goes over our router and that's it.


    Thanks for any insights,


  • 2.  RE: How to Begin Troubleshooting Slow Network Issues
    Best Answer

    Posted 05-16-2017 15:05

    I've been in networking now for 38 years and I have that 'slow network performance' is 99.9% related to packet loss/drops.  Some where in your path packets are being dropped, maybe via congestion or something similar.  Or bad HW, etc.


    1st step is probably to look at the stats of the switches to see if they show any errors, dropped packets, etc.  You could also look at the traffic via packet capture.  Look at it via port mirror to an analyzer.  Then you need to read the capture(s) along the path.


    Maybe easier said then done.  Hopefully stats show you something.

  • 3.  RE: How to Begin Troubleshooting Slow Network Issues

    Posted 05-17-2017 14:37

    Thanks for your response.  I did discover that my router is dropping packets when we're up around 1 Gbps, and it confuses me that packets are being routed when they're not crossing VLANs.  Do you have any ideas why that would be?  The router is Vyatta based.  But not sure sharing configs now would help.

  • 4.  RE: How to Begin Troubleshooting Slow Network Issues

    Posted 05-20-2017 06:34

    To know for sure we would need to see the topology of the connections between the host and computer being backed up.  


    But from your description, I assume the router in question is in the layer 2 path between the two ports involved in the communication.  Check the data sheet for the router for the bandwidth limitations.  You may be hitting a maximum for the device.


    The other thing to check since the limit is at GE level is if the path from port to port passes GE only restriction some where.  Or even a GE trunk port there this traffic plus the other traffic at the time is over capacity of the link as a result.


    And as noted above look for error counters on all the interfaces in the path to confirm there are no physical media problems.

  • 5.  RE: How to Begin Troubleshooting Slow Network Issues

    Posted 05-24-2017 18:27

    Really would need to see your topology and configs. 

  • 6.  RE: How to Begin Troubleshooting Slow Network Issues

    Posted 05-17-2017 13:43

    As already stated I would look for errors on both sides of the like Cisco and Juniper. If the uplink is a dot1q trunk make sure you turn off dtp and switch port negotiation on the cisco side. I've seen interface erros from not doing this. 


    After that look for obvious erros via Wireshark on whatever host is having the problems. 


    You can manually trace the path of things if you need to to by looking at routing and mac address tables. 




    Also, listen to 

  • 7.  RE: How to Begin Troubleshooting Slow Network Issues

    Posted 05-21-2017 22:03


    For slow network issues I would approach

    1) Check link drops , errors, CRC, Physical Layer.
    2) Check CPU utilization on both peers.
    3) Check STP state. (See if there is any fluctuation)
    4) Check COS/QOS settings.
    5) See if there are multiple next hops to reach the destination.
    6) Check for redundant links and try disabling them and check.

    This would be the start...