Question about Cisco Catalyst 3750G-24T SMI (WS-C3750G-24T-S) 24x1000 Mbps Networking Switch

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Unexplained network problems.

Our network core is all Cisco 3750G-24T switches. They had no configuration, the original IOS (c3750-i9-mz.121.14-EA1a) and we just plugged them in and they worked. In the past few months, we've been having intermittent network issues. Half the building would have horrible latency or won't be able to access the network at all. Looking at bandwidth graphs shows you that not more than 10 megabits of our gigabit connection between switches is used. We've put passwords on the switches, reconfigured all links to ensure they are gigabit and to reduce hops between devices. We've also updated the IOS on one switch as a pilot and are running Cisco network assistant to visualize the network. We have yet to identify what was happening, or determine if it will happen again. What does your organization do to prevent this kind of problem? Thanks in advance for the advice!

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  • joeyd666 May 27, 2009

    Hi Brad,

    Thanks for taking the time to check this out. Currently we have 1 VLAN and all users are on it. Also, the included image is our network. the caution and emergency icons are not problems. they have been resolved but the log of them was not removed.

  • joeyd666 May 27, 2009

    I uploaded a higher resolution graphic of our network and included show tech output.

  • joeyd666 May 29, 2009


    Sorry for the delay. we have approximately 200 users at any given time on a single VLAN. Unfortunately, Cisco Network Assistant is the only program we have that will map out our devices at the moment. Our Layer 3 is a single class B subnet at the affected site. There should be no routing other than with our default gateway.

    As for frequency, this occurs several times a month and lasts anywhere from 15 minutes to several hours. We've been unable to determine any abnormal CPU load or Bandwidth usage during an event. In fact bandwidth is often below 10Mbps during these situations according to cisco's stats. We also have a machine running NTOP on the main switch monitoring VLAN1 and that sees no major bandwidth usage over 15mbps most times. Most of our links are gigabit.

    You've brought up a few other good questions and I will see what I can do to answer them ASAP. In the mean time, can you suggest a sniffer if you don't use NTOP?

  • joeyd666 Jun 12, 2009

    FYI, I believe we found the problem. checking the logs again while experiencing the issue, we noticed a lot of host flapping. We were unable to resolve why this was occuring until Cisco Network Assistant showed a network loop for approximately 5 seconds. We were able to get the port numbers and located an old WAP11 everyone had forgotten about. Somehow it was malfunctioning and acting as a bridge, causing a network loop. This device was at the far end of the building so it only connected with another wireless device sporadically. Half of the building was going from 1Gbps to 11Mbps, causing major congestion.



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I would be glad to help with this.
How many VLANs do you have? How many users per VLAN? Do you have a generic diagram (stripped of all company info) that you can share? Can you send me a "sh tech" from the core switch so I have a better feel for how things are configured?

Posted on May 27, 2009

  • bradhscherer
    bradhscherer May 28, 2009

    Hey Joey,

    Thanks for all the detail. I have a few questions.
    How many users again on VLAN 1?

    Do you have a better diagram than Visualizer? I think I understand it but I hate the way Visualizer draws it. Do you have a Visio? Can you hack one together real quick? I need to see layer 2 & 3.

    How often is the latency?
    How long does it last?
    Do the logs on any device show anything?
    Can users ping their default gateway during latency?
    Can users ping through the default gateway?
    Can they ping each other on the same switch without latency?

    There are many many things that could be at play here. Including broadcast storms, rogue backups, virus scans, automated client patch rollouts, spanning tree, convergence, a device doing snmp queries to often etc.

    I would recommend upgrading to at least Maintenance Release 19 for your train of IOS to eliminate any software issues. It should be relatively painless.

    We use tools like Netscout Performance Manager, PRTG, Sniffers etc to keep a good eye on things. We have had many issues like this and more often than not it wasn't the network gear at all.

    The best thing to do is get a sniffer to analyze the traffic.

    Knowing more about the latency conditions and understanding your topology better will definitely help me help you better.
    And...dont worry you will get through this!!!

  • bradhscherer
    bradhscherer May 29, 2009

    Despite all the fancy sniffers and big money products....Wireshark is still my de facto sniffer and I use it all day everyday. It is very versatile and FREE!



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