Chp . 9: Troubleshooting

1. Chp. 9: Troubleshooting CCNA Discovery 2

2. Contents • 9.1: Troubleshooting approaches • 9.2: Troubleshooting Layer 1 and 2 • 9.3: Troubleshooting Layer 3 IP Addressing • 9.4: Troubleshooting Layer 3 Routing • 9.5: Troubleshooting Layer 4 & Upper Layers

3. Troubleshooting by Layers • One of the most important abilities for a network professional to develop is the ability to efficiently troubleshoot network problems • Knowing and Understanding the OSI Layers can help you Troubleshoot Networks • Logical networking models separate network functionality into modular layers. • Each layer of the OSI or TCP/IP model has specific functions and protocols.

4. Layered Models Network Applications File format, compression and encryption Manages communication sessions between applications Manages end-to-end message delivery, TCP provides reliability IP addressing , Routing of packets between networks Frames, MAC Addressing, Media Access Control Transmission of Bits over Media, Signaling, Encoding

5. Upper Layer Errors • The upper layers (5-7) of the OSI model deal with specific application functionality and are generally implemented only in software. • Problems in the Upper Layers (5 to 7) are frequently be caused by software configuration errors on clients and servers

6. Layer 3 and 4 Errors • The lower layers (1-4) of the OSI model handle data-transport issues. • Layers 3 and 4 are generally implemented only in software. • Problems in layers 3 and 4 are frequently caused by: • Software configuration errors on Clients • Router configuration errors • Firewall configuration errors • Layer 3 Problems are commonly caused by: • IP addressing and routing errors

7. Layer 1 and 2 Errors • Layer 1 and Layer 2 are implemented in both hardware and software. • The Physical Layer is closest to the physical network medium, such as the network cabling, and is responsible for actually placing information on the medium. • Most problems in Layer 1 and Layer 2 are caused by: • Hardware problems • Hardware incompatibilities

8. Technologies by Layer • It is important to understand where different Networking Technologies fit into the OSI Layers so you will know where to begin Troubleshooting • Upper Layers • Applications • Telnet • FTP • HTTP • DNS • DHCP • SMTP • SNMP • Layer 4 • TCP • UDP • Segments • Port Numbers • Sockets • Layer 3 • Routers • IP Addresses • Packets • Layer 2 • NICs • MAC Address • Frames • Switches • Layer 1 • Media • Hubs • Signals • NICs

9. Troubleshooting Approaches • There are 3 main troubleshooting approaches when using network models: • Top-down • Bottom-up • Divide-and-conquer • Using one of these troubleshooting methods, a troubleshooter can verify all functionality at each layer until the problem is located and isolated.

10. Top-down • The Top-down approach starts with the Application Layer and works down. • It looks at the problem from the point of view of the user and the application. • Is it just one application that is not functioning, or do all applications fail? • For example: • Can the user access various web pages on the Internet, but not email? • Do other workstations have similar issues? • Suitable for simpler problems or those where an application error is suspected

11. Bottom-up • The Bottom-up approach starts with the Physical Layer and works up. • The Physical Layer is concerned with hardware and wire connections. • Are cables securely connected? • If the equipment has indicator lights, are those lights on or off? • Suitable for more complex problems • Slow, but solid method

12. Divide-and-Conquer • The Divide-and-Conquer begins at one of the middle layers and works up or down from there. • For example: • The troubleshooter begins at the Network Layer by verifying IP configuration information • Next, they may work up or down the layers: • The functionality of the NIC is checked OR • Port Number Assignments are checked • Suitable for more complex problems and when the problem has less precise symptoms

13. Troubleshooting Approaches Problem: A customer reports that they are unable to view web pages from a web server located at the ISP • Bottom-Up Approach • The technician checks the cable connections between the web server and switch • The technician checks the lights on the NIC in the web server • Top-Down Approach: • The technician calls the customer to find out which apps are affected • The technician verifies that the DNS server has the correct DNS entry and is resolving the name • Divide-and-Conquer Approach: • The technician pings the server and then pings the customer’s switch • The technician checks the firewall configuration

14. Troubleshooting Tools • It is very difficult to troubleshoot any type of network connectivity issue without a network diagram that depicts the IP addresses, IP routes, and devices, such as firewalls and switches. • Logical and physical topologies are extremely useful in troubleshooting.

15. Physical Network Topology • A physical network topology shows the physical layout of the devices connected to the network. • Knowing how devices are physically connected is necessary for troubleshooting problems at the Physical Layer, such as cabling or hardware problems. • Physical network topologies typically include: • Device types • Models and manufacturers of devices • Locations • Operating system versions • Cable types and identifiers • Cabling endpoints

16. Physical Topology Diagram

17. Logical Network Topology • A logical network topology shows how data is transferred on the network. • Symbols are used to represent network elements such as routers, servers, hubs, hosts, and security devices. • Logical network topologies typically include: • Device identifiers • IP addresses and subnet masks • Interface identifiers • Routing protocols • Static and default routes • Data-link protocols • WAN technologies

18. Logical Topology Diagram

19. Troubleshooting tools • In addition to network diagrams, other tools may be needed to effectively troubleshoot network performance issues and failures. • Documentation and baseline tools • To create network diagrams and document hardware • Management system tools • To monitor network performance • Knowledge bases • Network device vendor knowledge bases are an indispensable sources of information. • Protocol analyzers • Decodes the protocol layers in a recorded frame and presents the information in an easy-to-use format.

20. Troubleshooting Hardware Tools Sometimes failures in the lower layers of the OSI model must be identified with hardware troubleshooting tools: • Cable testers • Handheld devices designed for testing data communication cabling for broken wires, crossed-over wiring, shorted connections, and improperly paired connections • Digital multimeters • Test instruments that measure electrical values of voltage, current, and resistance. • Portable network analyzers • For testing network traffic utilization

21. 9.2: Layer 1 and 2 Issues • The Physical and the Data Link Layers encompass both hardware and software functions. • All network communications rely on the technologies at Layers 1 and 2 in order to function • A network technician must be able to quickly isolate and correct problems occurring at these layers • To effectively troubleshoot Layer 1 and Layer 2 problems, technicians must be familiar with: • Cabling standards • Encapsulation • Framing

22. Layer 1 Issues • Layer 1 problems can result in loss of connectivity or degraded network performance • Symptoms of Layer 1 Issues: • Loss of connectivity • Network congestion • High collision count • Console error messages • Causes of Layer 1 Issues : • Bad, Improperly terminated or too long cables • Wrong cable type • Electrical or wireless Interference See Troubleshooting chart

23. Troubleshoot Layer 1 Issues • Most layer 1 issues are caused by Hardware problems and incompatibilities • To Troubleshoot Layer 1 Issues: • Visually Inspect Cables • Check Hardware Configurations • Use Show Interfaces to check Interface statistics • Monitor Network Performance

24. Layer 2 Issues • Symptoms of Layer 2 Issues: • Excessive Broadcasts • Intermittent Loss of connectivity • Excessive Collisions on an Interface • Console Error Messages • Causes of Layer 2 Issues: • Misconfigured Network cards • Faulty NIC drivers • Encapsulation errors on WAN links • Overly large Broadcast Domains

25. Troubleshoot Layer 2 Issues • Network analyzers can diagnose Layer 2 issues • To Troubleshoot Layer 2 Issues: • Check NIC drivers and configuration • Use Show Interfaces to check encapsulation • Monitor Network Performance with Network analzyers

26. Router Bootup • To troubleshoot device hardware failures and boot errors, you must understand the Router Boot Process: • Perform the POST and load the bootstrap program. • tests the router hardware • After POST, the bootstrap program is loaded • The Bootstrap program is loaded from ROm • Locate and load the Cisco IOS software • IOS is located from: flash memory (default), a TFTP server, or router Boots into ROMmon Mode and loaded into RAM • The IOS locates the hardware and software components and lists the results on the console terminal • Locate and load the startup configuration file or enter setup mode • Config file is loaded from: NVRAM, a TFTP server, or the router enters into setup mode • This starts routing processes and supplies addresses to interfaces

27. Router Bootup Process

28. Layer 1 & 2 Bootup Errors • POST failure • If POST fails, turn off the power, unplug the device, remove all interface modules., then reboot • Corrupt flash image • Some devices can load a boothelper image from ROM memory with limited capabilities. If no boothelper, the device will boot into ROMmon mode • Corrupt or missing configuration file • Use either a TFTP server to restore a backup file or manually re-configure the device • Configuration-Register Setting error • An incorrect configuration register setting tells the router the wrong location to look for the Cisco IOS image • From ROMmon mode, you can change the config-register setting back • Memory error • If there is not enough memory to load the image, you can usually boot into ROMmon mode with Ctrl-Break and load an older one • Module error • Faulty or improperly seated interface modules may not be recognized during the POST and Cisco IOS load. Remove them and reboot

29. Configuration Register • The configuration register setting on a router tells it where to look for the IOS file to boot from • The normal, default config-register setting is: 0x2102 • This indicates a normal Flash boot and tells the router to use the default boot sequence: • Flash, TFTP Server, ROMmon mode • If the config-register is set to something other than 0x2102, this will cause the router to look somewhere other than FLASH memory for the IOS file, and it will usually not boot properly

30. Viewing the Config-Register • Once a router is booted, the ONLY command that can be used to view the config-register is show version • Router# show version

31. ROMmon Mode • If a router fails to boot a valid IOS file, it will boot up in ROM monitor (ROMmon) mode • ROMmon software is a simple command set stored in read only memory (ROM) • It can be used to troubleshoot boot errors and recover the router when the IOS is not present • Troubleshooting in ROMmon mode: • Look in flash memory for a valid IOS image • rommon 1> dir flash: • Once an IOS image is found, boot the image manually • rommon 1> boot flash:<IOS filename>

32. Troubleshooting Boot Errors • In order to troubleshoot Router bootup errors: • Observe console messages during the boot sequence • Use the show commands to verify operational status When boot problems cause a network outage, use substitution to replace the device with a known good device to restore services to end users

33. Troubleshoot Boot Errors • Show version • displays the version of the operating system and whether all interface hardware is recognized. • Show flash • displays the contents of the Flash memory, including the Cisco IOS image file. It also displays the amount of Flash memory currently being used and the amount of memory available. • Show ip interfaces brief • shows the operational status of the device interfaces and IP addresses assigned. • Show running-configuration and show startup-configuration • verify whether all the configuration commands were recognized during boot

34. Layer 1 & 2 Router Interface Errors • Router Interface errors are often the first symptom of Layer 1 and Layer 2 cabling or connectivity errors • Causes of Layer 1 Interface errors: • Cable Errors • Physical Interface Errors • Hardware Errors • Causes of Layer 2 Interface Errors: • Encapsulation Error • Clock Rate Error • Misconfigured IP Address • Interface not Enabled

35. Interface Issues Layer 1 Issues The interface cannot detect any signal on the line – due to a bad cable, no cable, interface error, or failed CSU/DSU on a serial interface The Interface is disabled due to hardware errors Layer 2 Issues There is an encapsulation error, a missing clock rate, misconfigured IP Address or the interface is not receiving keepalives The Interface has not been enabled with the no shutdown command

36. Layer 1 & 2 Interface Media Errors • 4 different types of Layer 1 and 2 Media Errors that can exist on a router interface • Excessive noise • Excessive collisions • Excessive runts • Late collisions

37. Excessive Noise • The Interface shows many CRC errors but not many collisions • Usually indicates a media or cable error • Common causes are electrical interference, loose or damaged connections, or incorrect cabling type

38. Excessive Collisions • Can only occur on half-duplex or shared-media Ethernet connections (when hubs in use) • Can be caused by Damaged cables

39. Excessive Runts • Run Frames are usually caused by Malfunctioning NICs • Can also be caused by Damaged Cables

40. Late Collisions • A properly designed and configured network should never have late collisions. • The most common cause is excessive cable lengths • Can also be caused by Duplex mismatches

41. Troubleshoot Interface Errors • To troubleshoot Interface Errors: • Use show interfaces to look at the interface statistics • Use show ip interface brief to check the status of interfaces

42. Show ip interfaces brief

43. Layer 2: LAN Connectivity Errors LAN troubleshooting usually centers around switches, because the majority of LAN users connect to the network via switch ports. • Causes of LAN Connectivity Errors: • Port hardware failures • Cable errors • Ports not Enabled • Duplex and/or Speed Mismatch

44. LAN Connectivity Troubleshooting Switch Troubleshooting: • Observe port LEDs to verify port functionality • Red or orange indicates an error • Verify cable connections • Verify configuration to ensure ports are enabled • Switch# show run • Switch# show interfaces • Verify duplex settings on ports • Switch# show interface port status

45. Layer 2: WAN Connectivity Errors Typically, WAN connectivity relies on equipment and media that is owned and managed by a telecommunications service provider (TSP) 2 Most Common WAN Connectivity Issues on Routers: • Clock rate errors • Mismatched Encapsulation or Timing Troubleshooting WAN connectivity issues: • Verify functionality of WAN Interfaces with show interfaces serial • Router#show interfaces serial You should also know the type of modem or CSU/DSU being used

46. 9.3: Layer 3 Issues • Layer 3 Issues are largely due to poorly designed and configured IP addressing schemes • At Layer 3, each packet must be identified with the source and destination addresses of the two end systems. • A device must be configured with an IP address to exchange messages using TCP/IP • To troubleshoot Layer 3 problems, you must be able to: • Determine the range of host addresses that belong to each individual IP network or subnet

47. Subnetting Example • The Network 192.168.1.0 /24 needs to be subnetted to create 6 usable Subnets • Class: C • Default Subnet Mask: 255.255.255.0 • # of Bits to borrow : 3 • Total Subnets Created: 8 • 2X(x = bits borrowed) • Usable Subnets Created: 6 • 2 X - 2 (x = bits borrowed) • Total Host Addresses per Subnet: 32 • 2y(y = host bits left over) • Usable Host Addresses per Subnet: 30 • 2 y - 2(y = host bits left over)

48. Subnetting Example Not Usable Not Usable

49. IP Addressing Scheme Network Address Not Usable = Network Address Not Usable=Broadcast Address

50. Subnet Practice Given the Network Address and Subnet Mask, Define: The Range of Host Addresses The Broadcast Address The Next Network Address 10 244 30 228 255 255 255 252 252= 11111100 Place value of last bit borrowed = 4 Subnets Increase by a value of 4: 10 244 30 10 244 30 10 244 30 10 244 30 Go to Section: 9.3.1.3 in Discovery 2 to Practice this Activity