Quick explanation: Switches connect the wired devices in a building, and routers connect your network to other networks, including the internet. Problems here show up as a port that won’t work, devices that can’t reach each other, a camera or phone with no power, or traffic going the wrong way. This guide explains the common problems and the checks technicians use to find them.
Before you change anything: save a copy of the current configuration, and make changes during a quiet time. A wrong change on a switch or router can disconnect many people at once.
Cisco commands below are for Cisco IOS and IOS XE switches and routers. Other brands have similar commands with different names.
Cisco switch problems
What it looks like: devices on some ports can’t connect, the network is slow for everyone, or a port light is off or amber.
Start with an overview of every port: whether it’s connected, its VLAN, speed and duplex.
show interfaces statusshow logging- notconnect: no link. Check the cable, the device and the port.
- err-disabled: the switch shut the port down on purpose. See switch port troubleshooting.
- Speed or duplex not what you expect (for example 100 Mbps or half duplex on a modern device): check the cable and the settings on both ends.
- The log often names the exact port and reason, such as a link flapping up and down or a security violation.
VLAN configuration issues
What it looks like: a device gets no address or the wrong address range, or can’t reach its gateway, after a port or switch change. VLANs split one physical network into separate groups, so a port in the wrong VLAN puts a device on the wrong network.
show vlan briefshow interfaces trunk- Access port in the wrong VLAN: check which VLAN the port is in against which one the device should be in.
- VLAN missing from a trunk: trunks carry many VLANs between switches. If the VLAN isn’t on the trunk’s allowed list on every hop, traffic stops there.
- Native VLAN mismatch: both ends of a trunk should use the same native VLAN. The switch log usually reports a mismatch.
- No gateway for the VLAN: each VLAN needs a router or layer-3 interface, and usually a DHCP server or relay.
Router connectivity
What it looks like: devices can reach each other but not the internet, or one site can’t reach another.
- From a computer, ping the router’s inside address (your default gateway). If that fails, the problem is between the computer and the router.
- Check the router’s WAN or outside interface: is it up, and does it have an address and a default route?
- On a Cisco router, check interface status and addresses:
show ip interface briefAn interface that says down / down has no physical link. up / down usually means the link is there but the connection protocol isn’t working, often a provider or settings issue.
Switch port troubleshooting
What it looks like: one wall jack or port doesn’t work, or works slowly.
show interfaces status err-disabledshow interfaces GigabitEthernet1/0/5- err-disabled: find the reason first (common ones are port security, BPDU guard when someone plugged in a small switch, or a link that kept flapping). Fix the cause, then reset the port with
shutdownfollowed byno shutdownon that interface. - Rising error counters (input errors, CRC) usually mean a bad cable, connector or wall jack. Try a known-good patch cable and another port.
- Test the cabling from the wall jack to the patch panel with a cable tester. See Ethernet termination.
Routing problems
What it looks like: some networks are reachable and others aren’t, or traffic takes a strange path.
show ip routetracert 10.20.30.1- Check that the router has a route to the destination network, and a default route (0.0.0.0/0) for everything else.
- Routing has to work in both directions. If A can reach B but replies never come back, check B’s side for a route back to A.
tracertortracerouteshows the last device the traffic reached, which is where to look next.
PoE failures
What it looks like: a camera, phone or Wi-Fi access point on a switch port has no power, keeps rebooting, or works with limited features. Power over Ethernet (PoE) sends power down the network cable.
show power inline- Power budget: each PoE switch has a total power limit. When it runs out, new devices don’t get power. The command above shows used and remaining power.
- PoE type: the device and switch must support the same standard. Common ones are 802.3af (up to 15.4 W per port at the switch), 802.3at or PoE+ (up to 30 W) and 802.3bt (up to 60 W or 90 W). A device that needs PoE+ may not start on an 802.3af port.
- Cable: PoE runs up to 100 meters on proper copper cable. Damaged cable, poor terminations or copper-clad aluminum cable can cause power problems.
- Try the device on another PoE port, or with a PoE injector, to tell a device fault from a switch fault.
How to confirm it’s fixed
- The port shows connected at the expected speed and full duplex, in the correct VLAN.
- Error counters stop increasing.
- The device gets the right address, reaches its gateway and the internet, and keeps working after a reboot.
- Save the working configuration once you’re sure (on Cisco,
copy running-config startup-config).
Frequently asked questions
What does err-disabled mean on a Cisco switch?
The switch shut the port down because it detected a problem, such as a port security violation or a loop. Find and fix the cause, then reset the port with shutdown and no shutdown.
Why does my PoE camera keep rebooting?
Common causes are a switch that’s out of PoE power budget, a port that supplies less power than the camera needs, or a long or damaged cable.
Why can’t my device reach the gateway after moving it to a new port?
The new port is probably in a different VLAN. Check the port’s VLAN against the device’s network.
Sources
- Cisco: IOS and IOS XE command references for interfaces, VLANs, routing and Power over Ethernet
- IEEE 802.3af, 802.3at and 802.3bt Power over Ethernet standards
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