Common Issues with Switches

Switches like this Cisco SG200-26 are suitable for small businesses and modest networking needs.

In a perfect world, all of the devices on your network would stay up and running indefinitely once they’re configured. You could just watch your log files churn out endless reports that everything is working fine and you would never get a call from anyone about their connection being down. However, you’d probably be out of a job pretty quickly if that was the case. Like most electronic devices, switches will need routine maintenance.

Troubleshooting Switches

Common Issues

Telnet is down: Did you configure Telnet or just decide to go with SSH? Did someone else in your department change the password? In some cases, another network administrator did make changes, reloaded it and lost the configuration. A device somewhere on your network may be blocking Telnet if you get this kind of output:

Switch# telnet 192.168.1.1

Trying 192.168.1.1 …Open

[Connection to 192.168.1.1 closed by foreign host]

I wouldn’t be surprised if your first thought is that there’s some kind of rogue on your network that’s intercepting your Telnet. Remember, Telnet is not all that secure. However, you should also check your Telnet settings because typos are always possible when managing the switch settings. Even if your co-workers say they haven’t touched it, you should check things out personally. A show run command will show you the switch configuration, which should show whether the vty lines have been configured to allow for Telnet.

Can’t ping the switch or can’t ping through the switch: If you can’t ping the switch, you should check the configuration for the IP addressing, including the default gateway and subnet mask. If you can’t ping devices that are supposed to be on the switch’s VLAN, check that the devices involved have unique IP addresses that are part of the same address range as the switch.

The interface is down: By default, each interface is going to be shut down. You can activate them by accessing each interface on your switch and using a no shut command. If that doesn’t activate them, check the cabling and connectors to ensure that they are securely connected and free of damage.

The interfaces can be set to one of three modes: trunk, access, or dynamic. Dynamic mode is the default and tells the switch to detect which setting is appropriate. Access is used for connections to an end device. You can check the current mode an interface is in by typing switchport mode ?.

The switch should automatically detect the duplex and speed for any cable plugged into it. Just to be on the safe side, though, most administrators hard-set the speed and duplex of each interface. To check settings on each interface, type in show interfaces, followed by the name of the interface you want to inspect.

Hardware Issues: There aren’t many things more irritating to a network administrator than a switch that intermittently goes down and always seems to work when they go to inspect it. Sometimes it will simply go down for good, which is still annoying but easier to troubleshoot. The usual method is to plug a known working device into an unused port on the switch. You can also bounce a port by using the shut and then the no shut command. Swapping out the Ethernet cabling is another useful step. Keeping documentation on hand is also a good idea to interpret the status lights on the switch.

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