At the heart of every network is a router.  It’s a bit like a traffic policeman, making sure that inbound and outbound traffic ends up on the correct network and regulating the traffic flow so it conforms with Quality of Service and maximises traffic flow.

An incorrectly configured router can seriously affect network performance and, if really badly off, stop it entirely. If you find yourself in that position, here are some hints on troubleshooting router problems.

There are several hardware problems that cause a router to not operate correctly:

• Power issues. If the router power supply is failing or otherwise faulty, then the router will exhibit strange behaviour, intermittent faults, faults that have no rhyme or reason, or it might just stop and start again randomly. Loss of configuration information is also a symptom.
• Hardware Issues. A router is a mini-computer, and internal components can fail. Run hardware diagnostics to see if you can isolate a fault.
• Age Issues.  Routers are like the rest of us. They will give years of good and stable service, then become slower and less flexible as they age. They may not be capable of dealing with some recent network protocols efficiently and effectively.
• Cable issues. If a connection has a faulty cable, this can also produce intermittent problems. Check your Ethernet cables to make sure they are properly connected to the correct connections and are not too long. If necessary, or in doubt, replace the cable.
• Bad modules (SFCs, GBICs). Check that these are properly installed, connections are good and that the ports they use are correctly configured.
• External problems. Sometimes it isn’t your fault. Your ISP might have a failure of some sort affecting you, it could be a congestion problem because of the time of day or a problem down the line. Check with your ISP before assuming that the router is faulty.

If there appear to be no physical or external issues, now is the time to start looking at the router configuration:

• Recent Changes. If your problems have appeared after recent changes to the router, perhaps a software upgrade, then that’s where to start looking. Check the configuration to make sure that it’s still valid and that there are no new setting that need to be set.
• Changes Needed.  Routers have firmware and software which needs to be kept up to date. Applying patches and software fixes may resolve issues. Remember to take a backup, or have a backup router available before doing the update.
• Age Issues. While “Backward compatibility” is the mantra, some older routers might not be able to deal with the latest network protocols as quickly and efficiently as recent routers. Make sure that your router is compatible with new technologies like Spine and Leaf or Software Defined Networks.

More Technical Issues

If all the steps above indicate that the router is fine and dandy, but you are still experiencing problems, then you need to dive deeper into the router configuration:

• Tracert and Ping. On an attached desktop, run Ping from the command prompt to ping a known website or two. That should tell you if the sites are reachable and of any transmission or latency problems. Tracert will also tell you of any slow or dead links in the route to the site. Your ISP will also assist with external tracing or telling you of any known Internet issues.
• Port Configuration. Check the configuration of the individual ports and make sure that the correct devices are connected to the correct ports.  You may also need to check switch ports are correctly configured and connected to the router interface.  This is particularly important for routers that have recently been updated or otherwise reconfigured.
• VLANS. Some applications, for example VoIP use VLANs. Make sure that the ports devices needing VLANs are attached to are correctly configured with the correct VLANS.
• Routing Tables. Check that they are valid. Changes elsewhere in the network might invalidate them or make routing non-optimal. Beware spanning trees.
• Masking. If you are using incorrect subnet masks, then you may exclude some devices from the network. Check your DHCP server to make sure address ranges are properly set, and that the router is correctly moving traffic between subnets. Be very aware of problems caused by mis-configured spanning trees.
• DNS. Get the DNS wrong and your traffic isn’t going anywhere. Check network connectivity to see if only some subnets are affected. If you are forwarding traffic check out that is happening properly.
• Firewall. A mis-configured firewall may be blocking traffic or routing it via a distant continent.
• Quality of Service. Some sites try to prioritise traffic, for example voice traffic has priority over standard data traffic. Check that the router is correctly configured to implement QoS.

Overall routers, particularly in an internal Lan to ISP scenario deal with traffic between two networks. Finding the issue is usually relatively simple. Hopefully these hints will tell you where to look.