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Random Device Network Loss

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  • Random Device Network Loss

    So I inhereted a network with a mixed switch environment and I'm having some strange issues and could use some suggestions. Here are the details:

    Network is setup with 8 VLANS. These VLAN's are defined on the 3Com (older) root switch. I specify the VLAN port and then have a network connection to a managed (defaulted) switch.. this has been in place for a while. Some of the devices are new Cisco 2960S's and others are also 3Com.. some are unmanaged.. either way .. this has been working long before I came on board.

    Now here is the problem that has started to occur a few months ago. I noticed that a device would occassionally drop "off" of the network .. but here is the kick.. the device can still communicate on the VLAN that it's on... so 192.168.7.x device can see other 7.x devices.. but not outside of the VLAN.. when I first began troubleshooting.. I was able to get this resolved by changing the device's IP address to a static address on the same VLAN.. and this worked.. but it seems that this happens much more frequently now.. and I thought that this might be a DHCP issue.. but the issue seems to randomly pick a single device .. including devices with a static IP address. It has gotten to the point that I will log into the root switch (3com) and reset the ARP table.. and that will get the device back up.. but it's really strange.. and is randomly effecting devices regardless of DHCP or VLAN.. it's not a DNS issue.. because I'm not able to ping even with IP address.. so .. I think I'm out of Ideas.. suggestions and questions welcomed.. Thanks in advance.

  • #2
    Re: Random Device Network Loss

    If you have reachability issues outside the vlan then that could be a routing issue. As anything destined outside that vlan requires a route lookup. Sometime routing loops can cause reachability issues such as intermittent access. This is normally true with dynamic routing protocols such as RIP and RIPv2 which are very slow to converge after a loop.

    Another thing to check for is duplicate arp entries in your arp cache on the switches. Normally this occurs if you assigned the same ip address to multiple devices.

    Another option is someone on the network is corrupting your arp table by sending in bogus mac address to ip mappings. All switches have a limit on how many mac addresses they can learn. Usually if someone floods the switch and max's out the cam table then new mac's can't be learned. Essential it acts like a hub and just floods the arp's everywhere.
    CCNA, CCNA-Security, CCNP
    CCIE Security (In Progress)


    • #3
      Re: Random Device Network Loss

      Hmm... those are good suggestions.. I will check it out and update you shortly.