Information About Troubleshooting the Software Configuration
Software Failure on a Switch
Switch software can be corrupted during an upgrade by downloading the incorrect file to the switch, and by deleting the image file. In all of these cases, the switch does not pass the power-on self-test (POST), and there is no connectivity.
Lost or Forgotten Password on a Switch
The default configuration for the switch allows an end user with physical access to the switch to recover from a lost password by interrupting the boot process during power-on and by entering a new password. These recovery procedures require that you have physical access to the switch.
![]() Note |
On these switches, a system administrator can disable some of the functionality of this feature by allowing an end user to reset a password only by agreeing to return to the default configuration. If you are an end user trying to reset a password when password recovery has been disabled, a status message reminds you to return to the default configuration during the recovery process. |
Power over Ethernet Ports
A Power over Ethernet (PoE) switch port automatically supplies power to one of these connected devices if the switch detects that there is no power on the circuit:
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a Cisco pre-standard powered device (such as a Cisco IP Phone or a Cisco Aironet Access Point)
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an IEEE 802.3af-compliant powered device
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an IEEE 802.3at-compliant powered device
A powered device can receive redundant power when it is connected to a PoE switch port and to an AC power source. The device does not receive redundant power when it is only connected to the PoE port.
After the switch detects a powered device, the switch determines the device power requirements and then grants or denies power to the device. The switch can also detect the real-time power consumption of the device by monitoring and policing the power usage.
Disabled Port Caused by Power Loss
If a powered device (such as a Cisco IP Phone 7910) that is connected to a PoE Switch port and powered by an AC power source loses power from the AC power source, the device might enter an error-disabled state. To recover from an error-disabled state, enter the shutdown interface configuration command, and then enter the no shutdown interface command. You can also configure automatic recovery on the Switch to recover from the error-disabled state.
On a Switch, the errdisable recovery cause loopback and the errdisable recovery interval seconds global configuration commands automatically take the interface out of the error-disabled state after the specified period of time.
Monitoring PoE Port Status
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show controllers power inline privileged EXEC command
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show power inline EXEC command
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debug ilpower privileged EXEC command
Disabled Port Caused by False Link-Up
If a Cisco powered device is connected to a port and you configure the port by using the power inline never interface configuration command, a false link-up can occur, placing the port into an error-disabled state. To take the port out of the error-disabled state, enter the shutdown and the no shutdown interface configuration commands.
You should not connect a Cisco powered device to a port that has been configured with the power inline never command.
Ping
The Switch supports IP ping, which you can use to test connectivity to remote hosts. Ping sends an echo request packet to an address and waits for a reply. Ping returns one of these responses:
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Normal response—The normal response (hostname is alive) occurs in 1 to 10 seconds, depending on network traffic.
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Destination does not respond—If the host does not respond, a no-answer message is returned.
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Unknown host—If the host does not exist, an unknown host message is returned.
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Destination unreachable—If the default gateway cannot reach the specified network, a destination-unreachable message is returned.
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Network or host unreachable—If there is no entry in the route table for the host or network, a network or host unreachable message is returned.
Layer 2 Traceroute
The Layer 2 traceroute feature allows the switch to identify the physical path that a packet takes from a source device to a destination device. Layer 2 traceroute supports only unicast source and destination MAC addresses. Traceroute finds the path by using the MAC address tables of the Switch in the path. When the Switch detects a device in the path that does not support Layer 2 traceroute, the Switch continues to send Layer 2 trace queries and lets them time out.
The Switch can only identify the path from the source device to the destination device. It cannot identify the path that a packet takes from source host to the source device or from the destination device to the destination host.
Layer 2 Traceroute Guidelines
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Cisco Discovery Protocol (CDP) must be enabled on all the devices in the network. For Layer 2 traceroute to function properly, do not disable CDP.
If any devices in the physical path are transparent to CDP, the switch cannot identify the path through these devices.
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A Switch is reachable from another Switch when you can test connectivity by using the ping privileged EXEC command. All Switch in the physical path must be reachable from each other.
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The maximum number of hops identified in the path is ten.
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You can enter the traceroute mac or the traceroute mac ip privileged EXEC command on a Switch that is not in the physical path from the source device to the destination device. All Switch in the path must be reachable from this switch.
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The traceroute mac command output shows the Layer 2 path only when the specified source and destination MAC addresses belong to the same VLAN. If you specify source and destination MAC addresses that belong to different VLANs, the Layer 2 path is not identified, and an error message appears.
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If you specify a multicast source or destination MAC address, the path is not identified, and an error message appears.
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If the source or destination MAC address belongs to multiple VLANs, you must specify the VLAN to which both the source and destination MAC addresses belong. If the VLAN is not specified, the path is not identified, and an error message appears.
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traceroute mac
ip command output shows the Layer 2 path when the specified
source and destination IP addresses belong to the same subnet. When you specify
the IP addresses, the
Switch
uses the Address Resolution Protocol (ARP) to associate the IP addresses with
the corresponding MAC addresses and the VLAN IDs.
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If an ARP entry exists for the specified IP address, the Switch uses the associated MAC address and identifies the physical path.
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If an ARP entry does not exist, the Switch sends an ARP query and tries to resolve the IP address. If the IP address is not resolved, the path is not identified, and an error message appears.
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When multiple devices are attached to one port through hubs (for example, multiple CDP neighbors are detected on a port), the Layer 2 traceroute feature is not supported. When more than one CDP neighbor is detected on a port, the Layer 2 path is not identified, and an error message appears.
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This feature is not supported in Token Ring VLANs.
IP Traceroute
You can use IP traceroute to identify the path that packets take through the network on a hop-by-hop basis. The command output displays all network layer (Layer 3) devices, such as routers, that the traffic passes through on the way to the destination.
Your Switch can participate as the source or destination of the traceroute privileged EXEC command and might or might not appear as a hop in the traceroute command output. If the Switch is the destination of the traceroute, it is displayed as the final destination in the traceroute output. Intermediate Switch do not show up in the traceroute output if they are only bridging the packet from one port to another within the same VLAN. However, if the intermediate Switch is a multilayer Switch that is routing a particular packet, this Switch shows up as a hop in the traceroute output.
The traceroute privileged EXEC command uses the Time To Live (TTL) field in the IP header to cause routers and servers to generate specific return messages. Traceroute starts by sending a User Datagram Protocol (UDP) datagram to the destination host with the TTL field set to 1. If a router finds a TTL value of 1 or 0, it drops the datagram and sends an Internet Control Message Protocol (ICMP) time-to-live-exceeded message to the sender. Traceroute finds the address of the first hop by examining the source address field of the ICMP time-to-live-exceeded message.
To identify the next hop, traceroute sends a UDP packet with a TTL value of 2. The first router decrements the TTL field by 1 and sends the datagram to the next router. The second router sees a TTL value of 1, discards the datagram, and returns the time-to-live-exceeded message to the source. This process continues until the TTL is incremented to a value large enough for the datagram to reach the destination host (or until the maximum TTL is reached).
To learn when a datagram reaches its destination, traceroute sets the UDP destination port number in the datagram to a very large value that the destination host is unlikely to be using. When a host receives a datagram destined to itself containing a destination port number that is unused locally, it sends an ICMP port-unreachable error to the source. Because all errors except port-unreachable errors come from intermediate hops, the receipt of a port-unreachable error means that this message was sent by the destination port.
Time Domain Reflector Guidelines
You can use the Time Domain Reflector (TDR) feature to diagnose and resolve cabling problems. When running TDR, a local device sends a signal through a cable and compares the reflected signal to the initial signal.
TDR is supported only on 10/100/1000 copper Ethernet ports. It is not supported on 10-Gigabit Ethernet ports and on SFP module ports.
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Open, broken, or cut twisted-pair wires—The wires are not connected to the wires from the remote device.
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Shorted twisted-pair wires—The wires are touching each other or the wires from the remote device. For example, a shorted twisted pair can occur if one wire of the twisted pair is soldered to the other wire.
If one of the twisted-pair wires is open, TDR can find the length at which the wire is open.
Use TDR to diagnose and resolve cabling problems in these situations:
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Replacing a Switch
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Setting up a wiring closet
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Troubleshooting a connection between two devices when a link cannot be established or when it is not operating properly
When you run TDR, the Switch reports accurate information in these situations:
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The cable for the gigabit link is a solid-core cable.
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The open-ended cable is not terminated.
When you run TDR, the Switch does not report accurate information in these situations:
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The cable for the gigabit link is a twisted-pair cable or is in series with a solid-core cable.
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The link is a 10-megabit or a 100-megabit link.
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The cable is a stranded cable.
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The link partner is a Cisco IP Phone.
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The link partner is not IEEE 802.3 compliant.
Debug Commands
![]() Caution |
Because debugging output is assigned high priority in the CPU process, it can render the system unusable. For this reason, use debug commands only to troubleshoot specific problems or during troubleshooting sessions with Cisco technical support staff. It is best to use debug commands during periods of lower network traffic and fewer users. Debugging during these periods decreases the likelihood that increased debug command processing overhead will affect system use. |
All debug commands are entered in privileged EXEC mode, and most debug commands take no arguments.
Onboard Failure Logging on the Switch
You can use the onboard failure logging (OBFL) feature to collect information about the Switch. The information includes uptime, temperature, and voltage information and helps Cisco technical support representatives to troubleshoot Switch problems. We recommend that you keep OBFL enabled and do not erase the data stored in the flash memory.
By default, OBFL is enabled. It collects information about the Switch and small form-factor pluggable (SFP) modules. The Switch stores this information in the flash memory:
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CLI commands—Record of the OBFL CLI commands that are entered on a standalone Switch.
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Environment data—Unique device identifier (UDI) information for a standalone Switch and for all the connected FRU devices: the product identification (PID), the version identification (VID), and the serial number.
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Message—Record of the hardware-related system messages generated by a standalone Switch .
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Power over Ethernet (PoE)—Record of the power consumption of PoE ports on a standalone Switch .
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Temperature—Temperature of a standalone Switch .
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Uptime data—Time when a standalone Switch starts, the reason the Switch restarts, and the length of time the Switch has been running since it last restarted.
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Voltage—System voltages of a standalone Switch .
You should manually set the system clock or configure it by using Network Time Protocol (NTP).
When the Switch is running, you can retrieve the OBFL data by using the show logging onboard privileged EXEC commands. If the Switch fails, contact your Cisco technical support representative to find out how to retrieve the data.
When an OBFL-enabled Switch is restarted, there is a 10-minute delay before logging of new data begins.
Possible Symptoms of High CPU Utilization
Excessive CPU utilization might result in these symptoms, but the symptoms might also result from other causes:
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Spanning tree topology changes
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EtherChannel links brought down due to loss of communication
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Failure to respond to management requests (ICMP ping, SNMP timeouts, slow Telnet or SSH sessions)
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UDLD flapping
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IP SLAs failures because of SLAs responses beyond an acceptable threshold
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DHCP or IEEE 802.1x failures if the switch does not forward or respond to requests
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Dropped packets or increased latency for packets routed in software
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BGP or OSPF routing topology changes
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HSRP flapping