# Cisco Networking/CCENT/Troubleshooting

This lesson covers troubleshooting.

## Objectives and Skills

Objectives and skills for the troubleshooting portion of Cisco CCENT certification include:
* Troubleshoot and correct common problems associated with IP addressing and host configurations
* Troubleshoot and resolve VLAN problems
** Identify that VLANs are configured
** Verify port membership is correct
** Correct IP address is configured
* Troubleshoot and resolve trunking problems on Cisco switches
** Verify correct trunk states
** Verify correct encapsulation is configured
** Correct VLANs are allowed
* Troubleshoot and resolve ACL issues
** Verify statistics
** Verify permitted networks
** Verify direction
*** Interface
* Troubleshoot and resolve Layer 1 problems
** Framing
** CRC
** Runts
** Giants
** Dropped Packets
** Late Collisions
** Input/Output errors

## Readings

* Wikipedia: Troubleshooting
* Cisco: Troubleshooting Ethernet
* Cisco: Troubleshooting Tools

## Multimedia

* YouTube: The Network Troubleshooting Process - CompTIA Network+ N10-005: 1.8
* YouTube: Switch and Routing Diagnostics - CompTIA Network+ N10-005: 2.1
* YouTube: Troubleshooting Switch Loops - CompTIA Network+ N10-005: 2.5
* YouTube: Troubleshooting Network Cabling - CompTIA Network+ N10-005: 2.5
* YouTube: Troubleshooting Port Configuration - CompTIA Network+ N10-005: 2.5
* YouTube: Troubleshooting VLAN Assignments - CompTIA Network+ N10-005: 2.5
* YouTube: Troubleshooting Bad Fiber Modules - CompTIA Network+ N10-005: 2.5
* YouTube: Troubleshooting Mismatched MTUs - CompTIA Network+ N10-005: 2.5
* YouTube: Troubleshooting Power Failures - CompTIA Network+ N10-005: 2.5
* YouTube: Troubleshooting Routing - CompTIA Network+ N10-005: 2.5
* YouTube: Troubleshooting Subnet Masks and Gateways - CompTIA Network+ N10-005: 2.5
* YouTube: Troubleshooting Duplicate IP Addresses - CompTIA Network+ N10-005: 2.5
* YouTube: Troubleshooting DNS - CompTIA Network+ N10-005: 2.5
* YouTube: Cisco Troubleshooting Exercise

## Examples
### IP Addressing
#### show interfaces

To display statistics for all interfaces configured on the router or access server, use the show interfaces command in privileged EXEC mode.

#### Review

* Review Cisco Networking/CCENT/Remote Management
* Review Cisco Networking/CCENT/Remote Management
* Review Cisco Networking/CCENT/Static Routing
* Review Cisco Networking/CCENT/Switching
* Review Cisco Networking/CCENT/IOS Basics
* Review Cisco Networking/CCENT/IOS Basics
* Review Cisco Networking/CCENT/Remote Management
* Review Cisco Networking/CCENT/Static Routing
* Review Cisco Networking/CCENT/Remote Management

### VLANs

* Review Cisco Networking/CCENT/VLANs
* Review Cisco Networking/CCENT/VLANs
* Review Cisco Networking/CCENT/VLANs

### Access Control Lists

* Review Cisco Networking/CCENT/Access Control Lists
* Review Cisco Networking/CCENT/Access Control Lists
* Review Cisco Networking/CCENT/Remote Management

### Layer 1 Problems

* Review #show interfaces
* Review Cisco Networking/CCENT/Static Routing

## Activities

* Troubleshoot and correct common problems associated with IP addressing and host configurations.
** Add two routers to a new GNS3 project and start the devices.
** Set the router global, password, interface, line, and remote management configurations to various matching and mismatched configurations.
** Practice verifying and troubleshooting the configurations using the following commands.
*** show interfaces
*** show ip interface
*** show ip interface brief
*** show ip route
*** show arp
*** show running-config
*** show startup-config
*** ping
*** trace / traceroute
*** telnet
*** ssh
* Troubleshoot and resolve VLAN and trunking problems.
** Add a router, an EtherSwitch router and four VPCS PCs to a new GNS3 project and start the devices.
** Set the VLANs, port membership, encapsulation, trunking, and IP addressing to various matching and mismatched configurations.
** Practice verifying and troubleshooting the configurations using the following commands.
*** show vlan
*** show interface trunk
*** show interfaces switchport
* Troubleshoot and resolve ACL issues.
** Add three routers and two VPCS PCs to a new GNS3 project and start the devices.
** Configure IP addressing and static or dynamic routing to connect all devices.
** Test the configuration using the following commands from the routers and the PCs. Test all router and PC addresses. All tests should be successful.
*** ping
*** trace
** Configure standard and extended ACLs to various matching and mismatched configurations.
** Practice verifying and troubleshooting the configurations using the following commands.
*** show access-lists
*** show ip access-lists
*** show ip interface
* Troubleshoot and resolve Layer 1 problems.
** Add two routers to a new GNS3 project and start the devices.
** Set the link between the devices to various matching and mismatched configurations.
** Practice verifying and troubleshooting Layer 1 problems using the following commands.
*** shutdown
*** no shutdown
*** show interfaces
*** show cdp neighbors

## Lesson Summary

* Troubleshooting is a logical, systematic search for the source of a problem so that it can be solved, and so the product or process can be made operational again.
* Troubleshooting requires identification of the malfunction(s) or symptoms within a system. Then, experience is commonly used to generate possible causes of the symptoms. Determining the most likely cause is a process of elimination - eliminating potential causes of a problem. Finally, troubleshooting requires confirmation that the solution restores the product or process to its working state.
* A basic principle in troubleshooting is to start from the simplest and most probable possible problems first.
* Serial substitution involves checking each component in a system one by one, substituting known good components for each potentially suspect one.
* Bisection involves separating a larger system into two or more subsystems to isolate and identify problems and causes.
* One of the core principles of troubleshooting is that reproducible problems can be reliably isolated and resolved.
* Intermittent problems are often the result of components that are thermally sensitive, because the resistance of a circuit varies with the temperature of the conductors in it.
* Troubleshooters must always consider the possibility that there is more than one fault causing a given system failure.
* Troubleshoot common problems associated with IP addressing and host configurations using the show interfaces, show ip interface, show ip interface brief, show ip route, show arp, show running-config, show startup-config, ping, trace / traceroute, telnet, and ssh commands.
* Troubleshoot VLAN and trunking problems using the show vlan, show interface trunk, and show interfaces switchport commands.
* Troubleshoot ACL issues using the show access-lists, show ip access-lists, and show ip interface commands.
* Troubleshoot Layer 1 problems using the show interfaces and show cdp neighbors commands.

## Key Terms
### CRC (cyclic redundancy check)

    An error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data.

### duplex mismatch

    A condition where two connected devices operate in different duplex modes, that is, one operates in half duplex while the other one operates in full duplex.

### error disabled (err-disabled)

    A error situation detected on a port, resulting in the software shutting down that port.

### giant

    An Ethernet frame that is longer than the IEEE standard 1,518 bytes.

### late collision

    A collision that occurs more than 64 octets into the frame.

### runt

    An Ethernet frame that is less than the IEEE 802.3's minimum length of 64 octets.

### up and up

    Refers to the two interface states of line status and protocol status both being enabled.

## Review Questions

* Troubleshooting is _____. Troubleshooting is a logical, systematic search for the source of a problem so that it can be solved, and so the product or process can be made operational again.
* Troubleshooting requires _____. Troubleshooting requires identification of the malfunction(s) or symptoms within a system. Then, experience is commonly used to generate possible causes of the symptoms. Determining the most likely cause is a process of elimination - eliminating potential causes of a problem. Finally, troubleshooting requires confirmation that the solution restores the product or process to its working state.
* A basic principle in troubleshooting is to _____. A basic principle in troubleshooting is to start from the simplest and most probable possible problems first.
* Serial substitution involves _____. Serial substitution involves checking each component in a system one by one, substituting known good components for each potentially suspect one.
* Bisection involves _____. Bisection involves separating a larger system into two or more subsystems to isolate and identify problems and causes.
* One of the core principles of troubleshooting is that _____. One of the core principles of troubleshooting is that reproducible problems can be reliably isolated and resolved.
* Intermittent problems are often the result of _____. Intermittent problems are often the result of components that are thermally sensitive, because the resistance of a circuit varies with the temperature of the conductors in it.
* Troubleshooters must always consider the possibility that _____. Troubleshooters must always consider the possibility that there is more than one fault causing a given system failure.
* Troubleshoot common problems associated with IP addressing and host configurations using the _____ commands. Troubleshoot common problems associated with IP addressing and host configurations using the show interfaces, show ip interface, show ip interface brief, show ip route, show arp, show running-config, show startup-config, ping, trace / traceroute, telnet, andssh commands.
* Troubleshoot VLAN and trunking problems using the _____ commands. Troubleshoot VLAN and trunking problems using the show vlan, show interface trunk, and show interfaces switchport commands.
* Troubleshoot ACL issues using the _____ commands. Troubleshoot ACL issues using the show access-lists, show ip access-lists, and show ip interface commands.
* Troubleshoot Layer 1 problems using the _____ commands. Troubleshoot Layer 1 problems using the show interfaces and show cdp neighbors commands.

## Assessments

* Flashcards: Quizlet: CCENT - Troubleshooting
* Quiz: Quizlet: CCENT - Troubleshooting

## See Also

* Pluralsight: Cisco Network Troubleshooting for Beginners
* TechRepublic: Get to Know the Cisco IOS Show Interfaces Command