Objective 4.7: Connectivity Issues

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Objective 4.7: Connectivity Issues
Objective 4.6: Troubleshooting Methodology

Objective 4.7: Given a scenario, troubleshoot common connectivity issues and select an appropriate solution

Physical issuesEdit


Crosstalk refers to any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. This can occur within the different pairs of wires in a cable and is mitigated by using twisted pair cabling. In a wireless environment, two different wireless access points that are broadcasting on channels too close together in frequency can reduce the quality of the connection between themselves and wireless users.

In telecommunications, crosstalk is often distinguishable as pieces of speech or signaling tones leaking from other people's connections. If the connection is analog, twisted pair cabling can often be used to reduce the effects of crosstalk. Alternatively, the signals can be converted to digital form, which is much less susceptible to crosstalk.

Near End crosstalkEdit




Open impedance mismatch (echo)Edit


Logical issuesEdit

Port speedEdit

Port duplex mismatchEdit

A duplex mismatch occurs when two devices are using different duplex settings. In this case, one device will try to transmit using full duplex, while the other will expect half duplex communications. By default, devices are configured to use autonegotiation to detect the correct duplex setting to use. If a duplex method cannot be agreed upon, devices should default to using half duplex. A duplex mismatch can occur in the following cases: • Both devices are configured to use different duplex settings. • Autonegotiation does not work correctly on one device. • One device is configured for autonegotiation and the other device is manually configured for full duplex.

Symptoms of a duplex mismatch include very slow network communications. Ping tests might appear to complete correctly, but normal communications work well below the expected speeds, even for half duplex communications.

Incorrect VLAN (Virtual Local Area Network)Edit

Incorrectly assigning a port to a VLAN might prevent a device from communicating through the switch.

Incorrect IP (Internet Protocol) addressEdit

Wrong gatewayEdit

If the gateway is configured incorrectly, remote clients can't access network resources, local hosts can't access the Internet and they can't access hosts outside the local subnet.

Wrong DNS (Domain Name System) serverEdit

Wrong subnet maskEdit

Issues that should be identified but escalatedEdit

Switching loopEdit

Routing loopEdit

Route problemsEdit

Proxy ARP (Address Resolution Protocol)Edit

Broadcast stormsEdit

Wireless issuesEdit

Interference (bleed, environmental factors)Edit

Incorrect encryptionEdit

Incorrect channelEdit

Incorrect frequencyEdit

ESSID (Extended Service Set Identifier) mismatchEdit

Standard mismatch (802.11 a/b/g/n/ac)Edit



Incorrect antenna placementEdit

« Network Management
Objective 4.7: Connectivity Issues
Objective 4.6: Troubleshooting Methodology