Home

Troubleshooting

1. Excessive collisions Collisions usually occur only on half duplex or shared media Ethernet connections Damaged cables can cause excessive collisions Excessive runt frames Malfunctioning NICs are the usual cause of runt frames but they can be caused by the same issues as excessive collisions Late collisions A properly designed and configured network should never have late collisions Excessive cable lengths are the most common cause Duplex mismatches can also be responsible A runt frame is an Ethernet frame that is less than the IEEE 802 3 minimum length of 64 octets a late collision is one that happens further into the packet than is allowed for by the protocol standard Troubleshooting WAN Typically WAN connectivity relies on equipment and media that is owned and managed by a telecommunications service provider TSP Because of this it is important for technicians to know how to troubleshoot the customer premises equipment and to communicate the results to the TSP e To successfully troubleshoot serial WAN connectivity problems itis important to know the type of modem or CSU DSU that is installed and how to place the device in a loopback state for testing A CSU DSU Channel Service Unit Data Service Unit is a digital interface device used to connect a Data Terminal Equipment device or DTE such as a router to a digital circuit such as a T1 line Troubleshooting WAN 1 2 The interface status li
2. Divide and Conquer Typically troubleshooting begins at one of the middle layers and works up or down from there For example the troubleshooter may begin at the Network Layer by verifying IP configuration information Network Topologies 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 pae 192 168 2 4 192 168 2 5 192 168 2 6 Department 192 168 1 1 Server 192 168 1 2 192 168 1 3 Ethernet 192 168 2 0 aroy 192 168 1 4 192 168 1 5 192 168 1 6 Classroom 1 192 168 1 7 192 168 1 8 Printer 192 168 1 9 Classroom 2 Router Firewall Ethernet Internet 192 168 1 0 Classroom 3 Troubleshooting Tools Network Documentation and Baseline Tools can be used to draw network diagrams keep network software and hardware documentation up to date and help to cost effectively measure baseline network bandwidth use These software tools often provide monitoring and reporting functions for establishing the network baseline Network Management System Tools They graphically display a physical view of the network devices If a failure occurs the tool can locate the source of the failure and determine whether it was caused by malware malicious activity or a failed device
3. Knowledge Bases Protocol or Packet Analyzers decodes the various protocol layers in a recorded frame and presents this information in a relatively easy to use format Protocol analyzers can capture network traffic for analysis The captured output can be filtered to view specific traffic or types of traffic based on certain criteria Layer 1 Symptomes Performance lower than baseline Loss of connectivity 7 Application High collision counts Network bottlenecks or congestion 2 Data Link E 1 Physical High CPU utilization rates Console error mes sages Layer 2 Symptomes No functionality or connectivity at the Network Layer or above Network operating below baseline performance levels Excessive broadcasts Console error messages LI 2 Data Link 7 Application 6 Presentation 1 Physical Intermittent loss of connectivity Failing UPS or power supply Loose cable Layer 1 amp 2 Sym pto mes Excessive collisions on an interface Duplex mismatch Too many hosts on a shared network segment Console message indicating a protocol is down Mo keepalive signals are being recelved Encapsulation mismatch Troubleshooting Network Devices When booting any Cisco networking device it is helpful to observe the console messages that appear during the boot sequence After the Cisco IOS software is loaded the tech
4. file cannot be found some Cisco devices execute an autoinstall utility This utility broadcasts a TFTP request for a configuration file Other devices immediately enter an initial configuration dialog known as the setup utility or setup mode Interfaces Status The output for the show ip interface brief command includes a summary of the device interfaces including the IP address and interface status protocol Up up status indicates normal operation and that both the media and the Layer 2 protocol are functional Down down status indicates that a connectivity or media problem exists Up down status indicates that the media is connected properly but that the Layer 2 protocol is not functioning or is misconfigured Common Layer 2 issues that can cause an up down output include Encapsulation is improperly configured No keepalives are received on the interface Media errors Occasionally media errors are not severe enough to cause the circuit to fail but do cause network performance issues The show interfaces command provides additional troubleshooting information to help identify these media errors Excessive Noise On Ethernet and serial interfaces the presence of many CRC errors but not many collisions is an indication of excessive noise CRC errors usually indicate a media or cable error Common causes include electrical interference loose or damaged connections or using the incorrect cabling type
5. 0 10100000 11100000 11000000 11100000 11100000 21100000 Hierarchical IP addressing scheme Hierarchical 172 16 0 0 16 IP Addressing ET Scheme 172 16 4 0 22 172 16 12 0 22 72 16 8 0 722 172 16 5 0 24 172 16 10 0 24 172 16 13 0 24 ia i Hierarchical IP addressing scheme If IP addressing is assigned in a random manner it is difficult to determine where a source or destination address is located Hierarchical IP addressing schemes offer many advantages including smaller routing tables that require less processing power However a poorly planned hierarchical network or a badly documented plan can create problems such as overlapping subnets or incorrectly configured subnet masks on devices An overlapping subnet occurs when the address range of two separate subnets include some of the same host or broadcast addresses Overlapping subnets do not always cause a complete network outage Dhcp Issues Subnet having too many hosts is when some hosts are unable to receive an IP address from the DHCP server Use the show ip dhcp binding command to check whether the DHCP server has available addresses Use the show ip dhcp conflict command to display all address conflicts recorded by the DHCP server If an address conflict is detected the address is removed from the pool and not assigned until an administrator resolves the conflict DHCP Broadcast Forward Be
6. HL C ELLE Be HREHLRB COSENZA Troubleshooting Working at a Small to Medium Business or ISP Chapter 9 Copyleft 2012 Vincenzo Bruno www vincenzobruno it Released under Creative Commons License 3 0 By Sa Cisco name logo and materials are Copyright Cisco Systems Inc Overview After completion of this chapter you should be able to Use the OSI model as a framework for troubleshooting network problems Identify and correct problems with hardware and operation at Layer 1 and Layer 2 Troubleshoot IP addressing problems including subnet mask host range errors DHCP and NAT issues dentify and correct problems with RIPv2 configuration and implementation Explain possible causes of problems occurring with user applications and how to recognize symptoms of DNS failures Create a plan to prepare to take the ICND1 examination in order to obtain a CCENT certification Troubleshooting approaches Top down 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 Do other workstations have similar issues Bottom up Starts with the Physical Layer and works up The Physical Layer is concerned with hardware and wire connections Are cables securely connected f the equipment has indicator lights are those lights on or off
7. cause routers normally do not forward broadcasts either the DHCP server must be on the same local network as the hosts or the router must be configured to relay the broadcast messages e A router can be configured to forward all broadcast packets including DHCP requests to a specific server using the ip helper address command This command allows a router to change the destination broadcast addresses within a packet to a specified unicast address Router config if ip helper address x x x x e Once this command is configured all broadcast packets will be forwarded to the server IP address specified in the command including DHCP requests Troubleshooting NAT It is critical that the correct interfaces are designated as the inside or outside interface for NAT In most NAT implementations the inside interface connects to the local network which uses private IP address space The outside interface connects to the public network usually the ISP Verify this configuration using the show running config interface command Use traceroute to determine the path the translated packets are taking and verify that the route is correct Troubleshooting Layer 3 routing e he primary tool to use when troubleshooting Layer 3 routing problems is the show ip route command This command displays all the routes the router uses to forward traffic e he routing table consists of route entries from the following sources Directly conne
8. cted networks Static routes e Dynamic routing protocols e Any time a routing problem is suspected use the show ip route command to ensure that all the expected routes are installed in the routing table Dynamic routing issues 192 168 1 0 24 192 168 2 0 24 172 20 1 0 30 Bifshow ip protocols Routing Protocol is rip Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Sending updates every 3D seconds next due in 26 seconds Invalid after 180 seconds hold down 180 flushed after 240 Redistributing i Interface Send Recy RIP ne chain SerialD Q D 2 2 Automatic network summarization 15 in effect Layer 4 Issues e Acommon indication of Layer 4 problems is users reporting that some web services especially video or audio are not reachable e Verify that the ports being permitted and denied by the firewall are the correct ones for the applications For a better understanding of which ports correspond to specific applications review the information on TCP UDP and ports Troubleshooting Upper Layers e It can be difficult to isolate problems to the upper layers especially if the client configuration does not reveal any obvious problems o determine that a network problem is with an upper layer function start by eliminating basic connectivity as the source of the problem Using the divide and conquer method of tro
9. g as expected Uu w N HB oO 192 255 255 192 655 255 192 255 192 255 192 255 192 255 192 255 255 192 255 255 192 255 168 168 168 FEER 168 255 168 255 168 255 166 168 168 1 0 24 255 0 1 0 27 255 224 1 32 255 224 1 64 1 96 27 255 224 1 128 27 255 224 1 160 27 255 224 1 192 27 255 224 1 224 27 255 255 224 727 27 255 224 Address Mask Address Mask Address Mask Address Mask Address Mask Address Mask Address Mask Address Mask Address Mask 11000000 11111111 11000000 11111111 11000000 11111111 11111111 11000000 11111111 11000000 11111111 11000000 11111111 11000000 11111111 11000000 11111111 11000000 11111111 10101000 11111111 10101000 1111111 10101000 11111111 10191000 11111111 10101000 11111111 10101000 11111111 10101000 11111111 10101000 11111111 10101000 11111111 Subnetting Review 11111111 Oo000001 11111111 OOo000001 Oooo0001 11111111 00000001 11111111 ooo0ggo01 11111111 OoOO00001 11111111 OOO00001 11111111 oOo000001 11111111 00000000 90000000 00000000 11100000 00100000 11100000 01000000 11100000 01100000 11100000 10000000 1110000
10. ne of the show interfaces serial command can display six possible problem states Serial x is down line protocol is down DTE mode When the router serial interface cannot detect any signal on the line it reports both the line and the Layer 2 protocol down Serial x is up line protocol is down DTE mode If the serial interface does not receive keepalives or if there is an encapsulation error the Layer 2 protocol is reported down Serial x is up line protocol is down DCE mode In cases where the router is providing the clock signal and a DCE cable is attached but no clock rate is configured the Layer 2 protocol is reported down Serial x is up line protocol is up looped It is common practice to place a circuit in a loopback condition to test connectivity If the serial interface receives its own signals back on the circuit it reports the line as looped Troubleshooting WAN 2 2 Serial x is administratively down line protocol is down An administratively down interface is one that is configured with the shutdown command Usually all that is needed to fix this condition is to enter the no shutdown command on the interface e fthe interface does not come up using the no shutdown command check the console messages for a duplicate IP address message e f a duplicate IP address exists correct the problem and issue the no shutdown command again Serial x is up line protocol is up The interface is operatin
11. nician can use commands to verify that the hardware and software are fully operational he show version command displays the version of the operating system and whether all interface hardware is recognized The show flash command 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 The show ip interfaces brief command shows the operational status of the device interfaces and IP addresses assigned e The show running configuration and show startup configuration commands verify whether all the configuration commands were recognized during the reload Troubleshooting Hardware 1841 LED Indicators on successful boot p o e sme SYS PWR Green Router has successfully booted up and the software is functional Slow steady blinking when system is booting or in the ROM monitor SYS ACT Green Blinking when packets are transmitted or received on any WAN or LAN interface or when monitoring system activity CF Blinking Green Flash memory is busy Do not remove the CompactFlash memory card when this light is on Startup problems f there is not enough memory to decompress the image the device scrolls error messages rapidly or constantly reboots e The device may be able to boot into ROMmon mode by issuing a Ctrl Break command during startup f a valid startup configuration
12. ubleshooting begin with verifying Layer 3 connectivity otep 1 Ping the host default gateway otep 2 Verify end to end connectivity otep 3 Verify the routing configuration otep 4 Ensure that NAT is working correctly Step 5 Check for firewall filter rules End of lesson

Troubleshooting

Contents

Download Pdf Manuals

Related Search

Related Contents