Looking for Looped Network Cables

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Possible Symptoms

  • Port on Switch gets disabled, if you move the cable to a different port the disabled port follows it.
  • Traffic lights are flashing constantly, normal activity should be kind of random with uplink and key devices flashing more. Loops will seem a little more in sync flashing across all the ports tagged in the same vlan
  • Ping 10.*.254.1, ping times are above 10 ms and erratic, probably with timed out pings, the issues will be the most noticeable on the network where the issue is.
  • Unable to bring up the login page of HP 1810/1910 switch(es)
    • Power cycling the switch will give you access but the switch will slow down again and lock out the switch again.

Types of Loops

  • Looped Switch – two ends of a cable are plugged into the same switch (this could be through wall jacks or directly into a switch
  • Bridged VLAN – cable bridges two vlans (usually through wall jacks) this can some times only show up as computers getting the an IP from the wrong vlan
  • BPDU Circular Loop – three or more switches connected in a circle. If there are shared vlans this will cause a loop because of duplicate ARP table records

Finding the Loop

  • Plug into the the POE Switch and manually configure your network:
    • IP: 10.*.200.254
    • Subnet: 255.255.255.0
    • Router: 10.*.200.1
  • Setup an constant ping to 10.*.254.1
  • Unplug the uplink from all the switches in the school except the POE switch you’re connected to
    • It is unlikely that a loop would be caused by a single POE switch
  • Ping should drop to sub 4 ms (probably under 1 ms)
  • Look at all the switches, the Layer 3 (HP 3800) should be fairly quiet on all ports
  • Have a look at each of HP 1810/1910s see if any of them sill have a lot of traffic. The only VLAN that might have a lot of traffic is the Student Network because they’ll continue to work even after being disconnected. 90% of Staff traffic goes to the Internet.
    • If one switch still has a lot of traffic leave it till last
  • Plug in a switch
    • Check the ping to make sure it isn’t creeping back up
    • Check to make sure the activities lights are still random
  • Flashing in sync means that there is probably a loop on that switch
    • Symptoms can slow to reappear if it’s on a corner of the network with little traffic
    • Repeat with each switch
  • Once it’s been isolated to a switch look at jacks that have dumb hubs connected to them first to see if there are any loops.
  • Unplug all of the cables (use ties to group them by vlan)
    • If you know which vlan only disconnect the cables from that vlan
  • Plug the cables in one at a time until you find the culprit
    • Symptoms can slow to reappear if it’s on a corner of the network with little traffic