N10-009 Network Troubleshooting Study Guide for the CompTIA Network+

Network Service Issues

Network service issues are problems that affect the ability of a network to function properly, potentially impacting performance, connectivity, or network access. Given a scenario, you should be able to identify and troubleshoot common network service issues.

Switching Issues

Switches control the flow of traffic between devices on a network based on media access control (MAC) address and port assignments. Issues with switching can cause major disruption within a network.

Spanning Tree Protocol (STP)

STP is a Layer 2 protocol that is used to prevent network loops, which are when data packets continually flow through a network due to multiple available paths to the destination port. STP uses a logical topology of network devices, identifying and removing redundant paths.

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Root Bridge Selection

Root bridge selection is the identification and assignment of a primary switch as the central point within a network. The root bridge is used by STP to designate paths to other devices in the network, eliminating loops.

Port Roles

Ports in STP can be assigned roles, such as a root port, a designated port, or a non-designated port. The root port is the port that provides the best path to the root bridge. A designated port is a port that is used for communications from a network segment to the root port. A non-designated port is one that is blocked to prevent loops. How traffic travels through the network is based on the assigned roles of the port on a switch.

Port States

A port state in STP is the operational status of a port on a switch. There are four primary port states: blocking, listening, learning, and forwarding. In the blocking state, the port does not forward packets but monitors for bridge protocol data units (BDPUs), which are data packets used to exchange network topology and connection information. In the listening state, the port actively exchanges BDPU packets to identify network connections, but it still does not forward packets. In the learning state, the port actively learns MAC addresses but still does not forward packets. In the forwarding state, the port actively forwards packets through the network.

Incorrect VLAN Assignment

VLAN assignments are used to identify the group or network segment a port is assigned to, along with the permissions associated with the VLAN. When a port is assigned to the incorrect VLAN, network traffic is unable to communicate properly. Incorrect VLAN assignment is a common issue when a single device is unable to communicate properly.

Access Control Lists (ACLs)

ACLs are used abundantly throughout a network and control the accessibility of a resource by network entities. If a mistake is made in an ACL, it can cause communication disruptions that may extend throughout a network. For example, if a designated port is assigned to an ACL as a blocked port, it would result in the blocking of all traffic that uses the designated port.

Route Selection

A routing protocol specifies the method of communication between routers, which is used to discover and distribute network information. This information is then placed into a routing table and used to identify the most effective route a packet should take to reach its intended destination. Proper route selection and configuration is required to prevent routing loops, which occur when packets continuously hop between routers without reaching the intended destination.

Static routing is a routing protocol that uses preconfigured routing data to route packets to their destination. A distance-vector routing protocol bases routing decisions on destination routes and hop count only. A link-state routing protocol evaluates the destination route and hop count. It also creates and maintains neighbor tables, which store information on all routers that can share routing data, and topology tables, which contain information on all destination networks plus neighbors, creating a broad map of the entire internetwork. A hybrid routing protocol uses a mixture of characteristics from both link-state and distance-vector protocols.

Routing Table

A routing table is a collection of destination IP addresses with the best available routes to reach the IP address. Routing tables are stored locally and include information such as destination IP and next hop address. Routing tables can be compiled manually or using a route discovery protocol.

Default Routes

The default route is the route packets take when the route to the destination is unknown. If no default route is set and the destination is unknown, the packet will be dropped.

Address Pool Exhaustion

Address pool exhaustion occurs when the number of available IP addresses is exceeded. It is most commonly associated with the DHCP protocol. When all available addresses (the DHCP scope) are in use, this results in DHCP scope exhaustion. When an address pool is exhausted, new devices attempting to join a network will not be assigned an IP address and will be unable to communicate with the network.

Incorrect Default Gateway

A default gateway is the primary point of communication for all devices on a connected local network. All packets entering and exiting the network route through the default gateway. An incorrect default gateway occurs when the default gateway assigned is wrong, preventing or delaying connection to or communication with a network. Incorrect default gateway assignment may be a result of an error in manual entry, a router broadcast misconfiguration, or changes in the network’s settings.

Incorrect IP Address

An incorrect IP address means there is an error in the IP address assigned to a device. An incorrect IP address may be the result of manual entry errors such as invalid characters, digits, or syntax.

Duplicate IP Address

A duplicate IP address is a form of incorrect IP address that occurs when more than one device is assigned the same IP address, resulting in routing and connection issues.

Incorrect Subnet Mask

The subnet mask is the identifying number used to separate the host and network portion of an IP address. For example, 192.168.1.0/24 has a subnet mask of 255.255.255.000. If the subnet mask is incorrect, the IP address will attempt to route to the wrong subnet.

Performance Issues

Performance refers to the ability of a network to function and includes metrics such as speed, reliability, connectivity, and efficiency. For the Network+ N10-009 exam, given a scenario, you should be able to troubleshoot common performance issues.

Congestion/Contention

Congestion occurs when an excessive amount of data is attempting to traverse a network at a given time, whereas contention occurs when an excessive amount of traffic is attempting to access the same link or resource at the same time. Congestion and contention may cause performance issues such as dropped packets and delays.

Bottlenecking

Bottlenecking (or a bottleneck) occurs when the physical infrastructure of a network is unable to properly accommodate the traffic. Bottlenecking can cause performance issues, which may be temporary due to an unexpected increase in traffic or a continual problem requiring an upgraded or adjusted infrastructure.

Bandwidth

Bandwidth is the theoretical maximum amount of data that can be transferred over a given time period. Data traversing a network is limited by the network’s available bandwidth.

Throughput Capacity

Throughput is the amount of data that can move through a network successfully at a given time. Throughput is dependent on multiple factors, both physical and logical. At the physical level, throughput is limited by the medium used for transmission, such as the cabling type or the networking devices. At the logical level, throughput is limited by processing power limitations, protocol overhead, and application interactions.

Latency

Latency refers to the time it takes data to process and travel through a network. Increased latency may be a result of excessive security measures, such as adding extra rules to routers or continual auditing recording.

Packet Loss

Packet loss occurs when a packet fails to reach its intended destination. Packet loss can be a result of numerous network issues, including interference, weak signal strength, noise, or congestion.

Jitter

Jitter is the presence of inconsistent signal or connection to a network and is commonly seen in video or voice connections. Jitter may be a result of congestion, time drift, or routing issues.

Wireless

Wireless networks can create a host of potential problems, including numerous connectivity issues. Wireless connectivity can be affected by various factors, including the transmitting antenna, the channel used, and the location of the access points (APs), among others. Troubleshooting these common situations and considerations can provide useful information about the cause of wireless connection problems. Given a scenario, you must be able to identify and troubleshoot common wireless connectivity issues.

Interference

Interference in a radio signal can cause connection problems, including signal degradation, or it can even hamper the ability to connect at all. Interference is often caused due to signal bleed in which a competing signal is transmitted over the same frequency. Signal bleed is commonly caused by other wireless signals, such as Bluetooth signals, or other electronic devices, such as microwaves. Proper placement of APs is crucial in preventing or reducing interference.

Channel Overlap

Channel overlap is another common cause of signal interference. A channel is a smaller portion of a frequency band. For example, the 2.4 GHz frequency used by wireless APs is commonly divided into 11 channels, with channels 1, 6, and 11 never overlapping because of their distance from each other. Channel overlap commonly occurs in two ways, either through two wireless APs using the same band, such as the 2.4 GHz band, or by a single wireless AP using a channel that overlaps with other channels, such as channel 3.

Signal Degradation or Loss

Antenna cable degradation occurs when the physical cable connecting the device containing the wireless antenna exceeds optimal parameters, thereby creating signal degradation or even signal loss. For example, if a coax cable is used for the physical connection, signal degradation can occur if the length the signal must travel exceeds the distance the cable is able to properly transmit the signal.

Radio frequency (RF) attenuation or signal loss commonly occurs when the RF signal is unable to properly communicate with the receiving device, often due to the placement of the AP. For example, RF attenuation may occur if an AP is placed too far from the receiving device for it to create a strong connection or if a physical structure such as a wall is impeding the signal.

Insufficient Wireless Coverage

Insufficient wireless coverage often results in signal attenuation or an inability to connect to a wireless AP. Insufficient wireless coverage can be caused by a lack of APs transmitting in a location or by an excess number of receiving devices connecting to a single AP, overwhelming the available bandwidth on the AP. A site survey, a comprehensive overview of a wireless network including existing APs and signal coverage, and a heat map, a visual representation of wireless coverage, are useful tools for identifying wireless coverage areas, including insufficient coverage areas.

The type of antenna used by a wireless AP may also contribute to insufficient wireless coverage. How wireless signals are dispersed is dependent on the type of wireless antenna being used, either omni or directional, and the polarization of the antenna, which dictates the direction in which the signal leaves the antenna. Even the placement of the antennas themselves can affect the signals.

Client Disassociation Issues

Client disassociation occurs when a receiving device has difficulties maintaining connection and authentication to a wireless AP. Client disassociation can be caused by multiple factors, including signal loss, excessive distance from the AP, lack of available bandwidth on the connection, or interference.

Roaming Misconfiguration

Roaming occurs when a device automatically connects and disconnects from extended service set (ESS) wireless APs based on location, which maintains seamless connectivity. Misconfigurations in roaming settings may result in lost connection to the network. Roaming issues may be caused by too much or too little signal coverage, reauthentication configurations, or the wrong SSIDs.