Loop The Loop

A notoriously difficult problem for an IT support engineer to diagnose, trace and resolve is that of a loop in an Ethernet network.  The symptoms will often baffle even the most savvy of network support technicians, and it’s only when you’ve experienced the problem first hand that you can easily recognise and resolve it in the future.

The first sign that your network has a loop condition is often users complaining of poor network performance or lack of connectivity altogether.  You’ll probably see regular packet timeouts when pinging devices on the network affected by the loop, if not a complete lack of response altogether.  When trying to connect to the remote management console of network switches you’ll probably find them slow and unresponsive, that is if you can connect to them at all.  Looking at the switches physically the activity lights are likely to be going crazy, i.e. a broadcast storm.  Some switches support various different modes for the lights; make sure the mode is set to “Activity”.

Whilst not an absolute, should all of the above symptoms be present to some extent then you almost certainly have a network loop, i.e. one port on a switch is connected to another port on the same switch by some means, or there are dual paths between two switches on the network.  It could be that a user has inadvertently patched one floor port to another floor port, that a mistake has been made by a network technician in the patch cabinet, or somebody has made a malicious attempt to bring down the network.

Whatever the case, steps need to be taken to find the loop urgently.  This can be incredibly time consuming, as the only real means of success is to pull each cable out of each switch one at a time and watch for the network activity to revert to normal levels.  However, there is a much faster solution which will not only remove the loop from the network but will also prevent all loop conditions in the future.  Provided your switch supports it, enable Spanning Tree Protocol (STP) on each switch.  If your switch does not appear to run STP, it often will following a firmware update.  STP works by monitoring the network for loop conditions, and shutting down ports that are responsible for this undesirable situation.

Regardless of whether you have a loop in your network, consider enabling STP anyway as it could well save a few headaches in the future.

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