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.

The default configuration for the device allows an end user with physical access to the device 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 device.

Note

On these devices 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.

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:

• a Cisco pre-standard powered device (such as a Cisco IP Phone or a Cisco Aironet Access Point)

• an IEEE 802.3af-compliant powered device

• 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.

For more information, see the "Configuring PoE" chapter in the .

The device 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:

• Normal response—The normal response (hostname is alive) occurs in 1 to 10 seconds, depending on network traffic.

• Destination does not respond—If the host does not respond, a no-answer message is returned.

• Unknown host—If the host does not exist, an unknown host message is returned.

• Destination unreachable—If the default gateway cannot reach the specified network, a destination-unreachable message is returned.

• 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.

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 device in the path. When the device detects a device in the path that does not support Layer 2 traceroute, the device continues to send Layer 2 trace queries and lets them time out.

The device 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.

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 device 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 device is the destination of the traceroute, it is displayed as the final destination in the traceroute output. Intermediate device 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 device is a multilayer device that is routing a particular packet, this device 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.

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.

Use TDR to diagnose and resolve cabling problems in these situations:

• Replacing a device

• Setting up a wiring closet

• Troubleshooting a connection between two devices when a link cannot be established or when it is not operating properly

When you run TDR, the device reports accurate information in these situations:

• The cable for the gigabit link is a solid-core cable.

• The open-ended cable is not terminated.

When you run TDR, the device does not report accurate information in these situations:

• The cable for the gigabit link is a twisted-pair cable or is in series with a solid-core cable.

• The link is a 10-megabit or a 100-megabit link.

• The cable is a stranded cable.

• The link partner is a Cisco IP Phone.

• The link partner is not IEEE 802.3 compliant.

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.

You can use the onboard failure logging (OBFL) feature to collect information about the device. The information includes uptime, temperature, and voltage information and helps Cisco technical support representatives to troubleshoot device 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 device and small form-factor pluggable (SFP) modules. The device stores this information in the flash memory:

• CLI commands—Record of the OBFL CLI commands that are entered on a standalone device.

• Environment data—Unique device identifier (UDI) information for a standalone device and for all the connected FRU devices: the product identification (PID), the version identification (VID), and the serial number.

• Message—Record of the hardware-related system messages generated by a standalone device.

• Power over Ethernet (PoE)—Record of the power consumption of PoE ports on a standalone device.

• Temperature—Temperature of a standalone device .

• Uptime data—Time when a standalone device starts, the reason the restarts, and the length of time the device has been running since it last restarted.

• Voltage—System voltages of a standalone device.

You should manually set the system clock or configure it by using Network Time Protocol (NTP).

When the device is running, you can retrieve the OBFL data by using the show logging onboard privileged EXEC commands. If the device fails, contact your Cisco technical support representative to find out how to retrieve the data.

When an OBFL-enabled device is restarted, there is a 10-minute delay before logging of new data begins.

Excessive CPU utilization might result in these symptoms, but the symptoms might also result from other causes:

• Spanning tree topology changes

• EtherChannel links brought down due to loss of communication

• Failure to respond to management requests (ICMP ping, SNMP timeouts, slow Telnet or SSH sessions)

• UDLD flapping

• IP SLAs failures because of SLAs responses beyond an acceptable threshold

• DHCP or IEEE 802.1x failures if the switch does not forward or respond to requests

This section describes how to recover from a failed command switch.

If you have not configured a standby command switch, and your command switch loses power or fails in some other way, management contact with the member switches is lost, and you must install a new command switch. However, connectivity between switches that are still connected is not affected, and the member switches forward packets as usual. You can manage the members as standalone switches through the console port, or, if they have IP addresses, through the other management interfaces.

You can prepare for a command switch failure by assigning an IP address to a member switch or another switch that is command-capable, making a note of the command-switch password, and cabling your cluster to provide redundant connectivity between the member switches and the replacement command switch. These sections describe two solutions for replacing a failed command switch:

• Replacing a Failed Command Switch with a Cluster Member

• Replacing a Failed Command Switch with Another Switch

These recovery procedures require that you have physical access to the switch. For information on command-capable switches, see the release notes.

The IEEE 802.3ab autonegotiation protocol manages the device settings for speed (10 Mb/s, 100 Mb/s, and 1000 Mb/s, excluding SFP module ports) and duplex (half or full). There are situations when this protocol can incorrectly align these settings, reducing performance. A mismatch occurs under these circumstances:

• A manually set speed or duplex parameter is different from the manually set speed or duplex parameter on the connected port.

• A port is set to autonegotiate, and the connected port is set to full duplex with no autonegotiation.

To maximize device performance and ensure a link, follow one of these guidelines when changing the settings for duplex and speed:

• Let both ports autonegotiate both speed and duplex.

• Manually set the speed and duplex parameters for the ports on both ends of the connection.

Note	If a remote device does not autonegotiate, configure the duplex settings on the two ports to match. The speed parameter can adjust itself even if the connected port does not autonegotiate.

Cisco small form-factor pluggable (SFP) modules have a serial EEPROM that contains the module serial number, the vendor name and ID, a unique security code, and cyclic redundancy check (CRC). When an SFP module is inserted in the device, the device software reads the EEPROM to verify the serial number, vendor name and vendor ID, and recompute the security code and CRC. If the serial number, the vendor name or vendor ID, the security code, or CRC is invalid, the software generates a security error message and places the interface in an error-disabled state.

Note	The security error message references the GBIC_SECURITY facility. The device supports SFP modules and does not support GBIC modules. Although the error message text refers to GBIC interfaces and modules, the security messages actually refer to the SFP modules and module interfaces.

If you are using a non-Cisco SFP module, remove the SFP module from the device, and replace it with a Cisco module. After inserting a Cisco SFP module, use the errdisable recovery cause gbic-invalid global configuration command to verify the port status, and enter a time interval for recovering from the error-disabled state. After the elapsed interval, the device brings the interface out of the error-disabled state and retries the operation. For more information about the errdisable recovery command, see the command reference for this release.

If the module is identified as a Cisco SFP module, but the system is unable to read vendor-data information to verify its accuracy, an SFP module error message is generated. In this case, you should remove and reinsert the SFP module. If it continues to fail, the SFP module might be defective.

The device monitors the temperature conditions and uses the temperature information to control the fans.

Use the show env temperature status privileged EXEC command to display the temperature value, state, and thresholds. The temperature value is the temperature in the device (not the external temperature).

You can monitor the physical path that a packet takes from a source device to a destination device by using one of these privileged EXEC commands:

To run TDR, enter the test cable-diagnostics tdr interface interface-id privileged EXEC command.

To display the results, enter the show cable-diagnostics tdr interface interface-id privileged EXEC command.

The output from the show platform forward privileged EXEC command provides some useful information about the forwarding results if a packet entering an interface is sent through the system. Depending upon the parameters entered about the packet, the output provides lookup table results and port maps used to calculate forwarding destinations, bitmaps, and egress information.

Most of the information in the output from the command is useful mainly for technical support personnel, who have access to detailed information about the device application-specific integrated circuits (ASICs). However, packet forwarding information can also be helpful in troubleshooting.