Steps to Diagnose a Problem
Common Layer 1 Troubleshooting Issues
Common Layer 2 Troubleshooting Issues
Common Layer 3 Troubleshooting Issues
Wireless Troubleshooting
100
This occurs based on troubleshooter's knowledge of a network and his interrogation of information gathered.
What is to eliminate potential causes.
100
This should be considered due to EMI, especially, when it comes to copper cables.
What is cable placement.
100
Your Ethernet switch will not work at all if this occurs. To ensure Ethernet switches continue to work, you may want to equip them with an UPS.
What is a power failure.
100
This causes traffic from a host to not be forwarded off that host's local subnet.
What is an Incorrect Default Gateway.
100
Increased delay due to the use of CSMA/CA in WLANs, which introduces a random delay before transmitting data, in an attempt to avoid collisions or all wireless devices associated with a single wireless AP in the same collision domain, introducing the possibility of collisions (retransmissions), which can increase delay all contribute to this.
What is latency.
200
This occurs by using network maintenance tools or interviewing impacted users, for example.
What is collect information.
200
Consider this, due to digital transmission between devices. Degradation can occur and the receiving equipment will be unable to correctly interpret the transmissions.
What is distance limitations exceeded.
200
When this occurs the flexibility to connect a variety of media types and speeds to the switch is lost. Some examples include GBIC and SFP.
What is a Bad module.
200
An example would be a host unable to browse the Internet using domain names.
What is an Incorrect DNS configuration.
200
Examples of this include a client using a wireless standard not supported by the wireless AP; a wireless client and AP using different channels, encryption standards, or SSIDs.
What is misconfiguration of wireless parameters.
300
This occurs when the troubleshooter tests to confirm or refute his theory as to the problem's underlying cause.
What is verify hypothesis.
300
This occurs when an analog connection creates and electromagnetic field around its conductors, inducing its waveforms on a nearby analog connection. It can be minimized by using a higher category of cabling since it limits the radiation of electromagnetic waves.
What is crosstalk.
300
Common settings include speed, duplex, and MDIX. Mismatched parameters could result in slow communication or in no communication because of this.
What is Port configuration (misconfiguration).
300
This creates unpredictable traffic patterns in hosts when two host are configured the same.
What is Duplicate IP address
300
It is caused when a waveform of a wireless transmission encounters a conductive object and most of the signal bounces of the object; this bouncing can cause data (specifically, bits) to arrive at uneven intervals, possibly corrupting data.
What is multiple paths of propagation.
400
This is where comparing information against previously collected baseline information occurs.
What is examine collected information.
400
An example of this would be interconnecting two 1000BASE-TX devices using a Cat 5 Cable; therefore, resulting in corrupted data.
What is Bad cables or connectors.
400
Problems such as devices not being assigned IP addresses in the same subnet and traffic not flowing because it can't be routed are some results of this.
What is VLAN configuration (misconfiguration).
400
A router interface has a parameter set that defines the largest packet size the interface will forward; if the packet exceeds this setting the router drops the packet. The router could respond with an ICMP message if a DF (don't fragment) bit is set, but if it isn't set the router doesn't send the message causing a "black-hole router".
What is a Mismatched MTU.
400
Its value varies based on distance from a wireless antenna and physical objects interfering with line-of-sight communication with a wireless antenna; some network drop their transmission rate when this value drops below a certain value.
What is signal strength.
500
This occurs after multiple potential causes are eliminated and only one or more causes are likely to have resulted in the problem.
What is Hypothesize underlying cause.
500
When interconnecting network components that do not support MDIX (medium dependent interface crossover) this problem often occurs because you will have to use a crossover cable instead.
What is Transposed Tx/Rx leads.
500
Some results are MAC address table corruption, broadcast storms, or suboptimal functioning of STP because a root bridge was automatically selected, rather than being specified.
What is Layer 2 loop.
500
The result is a remote host being unreachable, due to a host sending an ARP request in an attempt to determine the MAC address of the destination host, rather than forwarding traffic to the sending host's default gateway.
What is an Incorrect subnet mask.
500
This occurs when wireless APs are not strategically placed in a building to provide sufficient coverage; the coverage between coverage areas are not maintained and you do not have the recommended overlapping coverage areas (using non-overlapping channels) overlapping of approximately 10-15%
What is Incorrect AP placement.
M
e
n
u