In this post, I’ll document various network-related problems I’ve encountered, as well as steps and tools to troubleshoot them.

Common Issues

Loops – a loop happens when, through however many hops, devices, and cables, a device ends up plugged into itself. This could happen when both ends of the same cable are plugged into two different ports on a device, or when a device is plugged into another device twice, or any series of connections that result in a loop. Although STP (Spanning Tree Protocol) is supposed to prevent this from causing a problem, we’ve had limited success simply relying on that.

The problem with loops arises when broadcast information is put onto the network – this is a message that should be received by the device and then sent out on all appropriate ports. The problem is when that broadcast comes back through the loop on the other side – the network device sees it as a new broadcast, and so broadcasts it again. This causes massive network congestion, and in our experience generally comes across as network instability – the connection drops randomly, or is sometimes slow, etc. It can be difficult to track down since it’s an inherently-physical problem that has to be handled in-person.

The easiest troubleshooting is probably to ask, “Was something new plugged into the switch/device, or were any changes made to what’s plugged in?” Occasionally this is enough to find the troublesome cable. Other basic troubleshooting is simply to unplug cables and then plug them back in until the issues comes back. Additionally, cables can be traced – either physically or with a cable tracer/toner (see next post) to see if the signal comes back on a different line. When the troublesome cable is identified it can be removed.

Network port covers/caps can be put into unused ports on a network device to make this harder to accidentally do in the future.

IP-Assignment Issues

This comes in several forms. Two of the more common are, first, DHCP Exhaustion, where there are more devices on the network asking for an IP address than the scope allows, and secondly, Duplicate IPs, where two devices have the same IP. The first can be resolved by lowering the lease time (so disconnected IPs are freed up quicker) or expanding the scope to allow more devices. The second issue, where two devices have the same IP, arises when one (or both) devices are set static – usually when one device is static, but its IP address is within the DHCP scope so it’s assigned to something else. The best way to avoid this is either to make sure static IPs are outside the scope, or (more safely) to add DHCP reservations even for devices that  are static (which has the added benefit of assigning the correct IP if the device somehow loses its static settings and reverts to a more factory-default DHCP mode).

Routing Issues

        Occasionally, an issue arises where a device simply fails to communicate with the destination. This can be caused by a bad subnet mask, where the device has a faulty “understanding” of what its local network is, or it can be caused by routing issues caused either by bad (or missing) static routes, or a Default Gateway that is not actually on the local network. This is resolved by putting the correct information in the device.

Tools Referenced Above

• Ipconfig – this returns the IP information for the device. With the -all flag, it will return ALL information to have a network connection, as well as the device MAC address and other features.
• Ping – this is used to test connectivity to a given IP address. With the -a flag, it will attempt to resolve the hostname of the IP. With the -t flag, it will ping continuously until interrupted, in contrast with normal ping which a set number of attempts.
• Tracert/traceroute – mentioned in a previous post, this will return a list of all IPs encountered between the source and destination.
• Arp – this will return a list of MAC addresses and matching IP addresses.
• Nslookup – this command will query DNS for the hostname of a given IP address.

Physical Tools:

Cable tester & toner – in contrast to the above, these are physical tools used to determine a cable type, length, design, and check for faults and other issues (in the case of a tester). A toner will put a signal on the cable that, when paired with a probe, will return an audible sound allowing you to match one end of the cable to the other.