Deck 20 - Troubleshooting Cabling and Physical Issues

Incorrect Cable Type

Using a cable type not suited to the specific application or environment; can lead to network failures, reduced performance, and connectivity problems. Ensuring the correct cable type is essential for maintaining network integrity.

Single Mode Fiber

Fiber optic cable with a smaller core, used for long-distance transmissions; supports higher bandwidth with less signal attenuation than multimode.

Multimode Fiber

Fiber optic cable with a larger core, suitable for shorter distances; allows multiple light modes, which can cause more signal dispersion and attenuation over longer distances.

Single Mode/Multimode Mismatch

Using multimode fiber where single mode is required (or vice versa); this mismatch can result in increased attenuation, poor signal quality, and reduced bandwidth.

Incorrect Cable Category (Cat 5/6/7/8)

Using a lower-category cable (e.g., Cat5) instead of a higher category (e.g., Cat6, Cat7, or Cat8) can limit data transfer speeds and lead to increased errors, causing network slowdowns, increased latency, and an inability to support high-speed or data-intensive applications.

Shielded Twisted Pair (STP) vs. Unshielded Twisted Pair (UTP)

STP cable has added shielding to protect against electromagnetic interference (EMI); UTP does not. Using UTP instead of STP in environments with high EMI can result in signal degradation and data corruption, leading to frequent retransmissions and reduced reliability.

Signal Degradation

Occurs when the quality of a signal diminishes over distance or due to interference, leading to poor network performance. Common causes include incorrect cable types, physical damage, and environmental factors such as electromagnetic interference (EMI) or radio frequency interference (RFI).

Crosstalk

A specific type of signal degradation where a signal transmitted on one cable or channel interferes with a signal on another cable or channel; caused by incorrect or low-quality cables, such as those with insufficient shielding or untwisted pairs.

Near-End Crosstalk (NEXT)

A type of crosstalk interference that is measured at the transmitting end of a cable.

Far-End Crosstalk (FEXT)

A type of crosstalk interference that is measured at the receiving end of a cable.

Interference

The disruption of signal transmission caused by electromagnetic signals from other electronic devices or cables; can lead to data corruption and loss of connectivity, affecting network performance.

Attenuation

The gradual loss of signal strength as it travels through a cable or medium; increases with distance and can affect the quality of the communication, requiring the use of repeaters or amplifiers to maintain signal integrity.

Improper Termination

Occurs when network cables are not correctly terminated with the appropriate connectors or techniques; issues include signal loss and reflection (leading to data transmission errors) and increased electromagnetic interference (EMI), causing further degradation of signal quality.

Transmitter (TX)/Receiver (RX) Swap

Happens when the transmitter and receiver wires are incorrectly connected (crossed), causing devices to be unable to establish a proper link, leading to a complete loss of communication between networked devices.

Network Interface Counters

An interface status monitor that tracks real-time data throughput and error metrics, where escalating counters serve as a diagnostic trigger for physical or configuration-layer issues.

Ex: show interfaces gigabitethernet 1/0/1

Cyclic Redundancy Check (CRC) Errors

Occur when there is a mismatch in the data checksum, indicating data corruption during transmission; caused by faulty cables, electromagnetic interference (EMI), or hardware failures, resulting in data retransmission, increased latency, and reduced network throughput.

Runts

Packets that are smaller than the minimum allowed size (usually less than 64 bytes).

Giants

Packets that exceed the maximum allowed size (usually greater than 1518 bytes for Ethernet frames).

Drops

Occur when packets are discarded due to congestion, buffer overflow, or configuration issues.

Err-Disabled

A port status where the port has been automatically shut down by the network device due to a detected issue, such as a security violation (e.g., a port security breach) or excessive errors/link flaps; resolved by identifying and resolving the underlying issue before re-enabling the port.

Administratively Down (port status)

A port status where the port has been manually disabled by a network administrator, typically for maintenance, configuration changes, or security reasons; the port can be re-enabled through administrative action once necessary changes or maintenance are completed.

Suspended (port status)

A port status where the port is temporarily disabled, usually due to network policies or dynamic configurations, such as policy enforcement (violation of network access controls) or dynamic adjustments by protocols like LACP; the port may automatically re-enable or require manual intervention.

Power over Ethernet (PoE)

A technology that allows network cables to carry electrical power, simplifying the installation of networked devices like IP cameras and wireless access points.

Power Budget Exceeded (PoE)

Occurs when the total power consumption of connected PoE devices exceeds the available power budget of the switch; symptoms include devices failing to power on or operating intermittently, resolved by reviewing and managing power requirements or upgrading the PoE switch.

Incorrect PoE Standard

Occurs when devices and switches adhere to different PoE standards (e.g., IEEE 802.3af, 802.3at, 802.3bt), resulting in compatibility issues such as devices not receiving power or receiving insufficient power; resolved by ensuring all devices and switches comply with the same PoE standard.

Mismatched Transceivers

Using incompatible transceivers (mismatched vendor or standard) that can lead to connectivity and performance issues, such as no link light, data errors, or intermittent connections; resolved by verifying transceivers are compatible with each other and the devices they're connected to, from the same vendor or meeting the same standards.

Transceiver Signal Strength Issues

Poor signal strength in transceivers can result in data transmission errors and reduced network performance, showing as high error rates, dropped packets, or no connectivity; resolved by checking and cleaning fiber connectors, ensuring proper cable length and quality, and verifying transceiver specifications.

802.3af

PoE (Type 1): Provides up to 15.4 Watts

802.3at

PoE+ (Type 2): Provides up to 30.00 Watts

802.3bt

PoE++ (Type 3): Provides up to 60.0 Watts

802.3bt

PoE++ (Type 4): Provides up to 90.0 Watts