Network Troubleshooting (5.0)

Network Protocol Analyzer (WIRESHARK)

BEST suited for capturing and analyzing real time network traffic to diagnose performance issues

allows you to inspect individual packets, analyze traffic flow, sources of latency, capturing packets for analysis, and monitoring network performance

Network Taps

Least likely to introduce significant performance degradation or packet loss in a network

creates a copy of the data packet for monitoring and analysis (without interrupting network flow)

LEAST COST EFFECTIVE for managing unexpected network congestion during peak usage

Expand the physical bandwidth to manage unexpected network congestion during peak usage

Malfunctioning NIC

Occurs when GIANTS are still present even after properly configuring the MTU size on an interface

PoE power budget exceeded

Some connected devices may stop receiving power

Devices will fail to power on or may operate intermittently

Switch port is not set to not forward traffic, but the MAC address table is still being populated with new addresses.

port is most likely in a learning state

the port learns MAC addresses to prepare for forwarding but does not yet forward frames

Incorrect default gateway configuration on a client device

The device can communicate with local network devices but cannot access the internet

Misconfigured routing protocols

most common cause of high latency in a network

Can cause data to take inefficient paths through the network (leads to increased latency)

show vlan

used to verify which switch ports are assigned to specific network segments or groups

ensures each port is correctly configured as intended

Transceiver mismatch

Would cause inconsistent network connectivity between devices

Outdated switch

You can tell if the switch is outdated if it is constantly running at a near maximum capacity.

It is unable to handle the current traffic load, creating a bottleneck

show interface cmd

provides details on interface status, error counters, traffic load, utilization stats, and duplex settings

does NOT show MAC addresses learned on the interface

lowering bridge priority value

increases the switches likelihood of becoming the root bridge

STP selects the switch with the lowest bridge priority value

MOST EFFECTIVE for identifying the root cause of intermittent network slowdowns

Conducting real time packet capture during the slowdown

More effective than NetFlow data as it gives the most immediate and detailed information

Suspended state on switch port

a port that has been configured incorrectly

where one or more member ports are not properly aligned with the rest

Network Contention/Congestion

The competition among devices or users for limited network resources

occurs when multiple devices try to access the same bandwidth or network infrastructure simultaneously

leads to reduced performance, slower speeds, or dropped connections

Common cause of packet loss

faulty network cables are a common cause of packet loss because physical damage or wear can disrupt the transmission of data leading to dropped packets

Decreasing TTL

allows changes to propagate through the network more quickly for a resource record

Link layer discovery protocol (LLDP)

can operate across different devices from different vendors (both CISCO and non CISCO)

ideal for a mixed environment where interoperability is required

used for dynamically discovering and advertising capabilities of network devices on a LAN

provides info about device capabilities, VLAN membership, management information, and more

(operates at layer 2)

“four pair” mode PoE

delivers power to all four pairs of wires in the cable

packet sniffer

can capture and analyze network user traffic

allows you to view websites and contents visited by a user

IP scanner

used to monitor a networks IP address space in real time

can identify any devices connected to the network

a tool that will ping every IP on the network and creates a report based off the IPs that respond

Jitter

variation in the delay of received packets

irregular arrival times causing intermittent disruptions (freezes or delays)

Packet loss

refers to data packets being lost in transit

causes gaps in audio or video (more severe than intermittent freezes or delays)

Latency

the delay between the sending and receiving of data packets

doesn’t directly cause intermittent disruptions

Optical Time Domain Reflectometer (OTDR)

used to certify the performance of new fiber optics links and detect problems with existing fiber links

can identify if a fiber cable is broken and provide an approximate location for the break in meters or feet

Light meter

used to measure the power in an optical signal over a fiber optic cable

can be be used to detect if the cable is broken but would not determine where the break is (unlike OTDR)

CRC errors

usually indicative of physical layer issues (cable problems, bad NICs, or mismatched duplex settings)

Multimeter

can measure voltage, resistance, and amperage of a cable or conduit

(measured in ohms)

Runts

usually caused by physical layer issues (duplex mismatches, cable faults, and network collisions)

Route cmd

used to create, view, or modify manual entries in the network routing tables of a computer or server

port status “Blocked” in STP

the port does not forward user traffic but listens for STP messages

maintains network topology information

What happens when using cat5 cables for devices that specify using cat6 cables

cat5 cables will limit the networks ability to support Gb speeds

leads to potential bottlenecks