Layer Problems
Symptoms of Problems Occurring at the Transport Layer
When the transport layer has problems, you can observe the symptoms at the application layer. Session layer protocols and network applications rely on the correct operation and services that the transport layer provides. With transport layer problems, users notice a lack of connectivity to network resources or network applications that do not function. The main transport layer protocols of the TCP IP suite are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). (Real-Time...
Resolving Problems at the Transport and Application Layers
Chapter 11, Isolating a Problem at the Transport or Application Layer, presented a set of common symptoms for each of the transport and application layer-related problems. Table 13-2 provides a summary of those symptoms for your review. Table 13-2 Common Symptoms of Transport and Application Layer Problems Table 13-2 Common Symptoms of Transport and Application Layer Problems Common Symptoms of Problems Occurring at the Transport Layer Common Symptoms of Problems Occurring at the Application...
Components of a Network Configuration Table
Network configuration tables store accurate information about the hardware and software components of a network. Recording data into these tables, referring to these tables to look up information, and maintaining the accuracy of these tables are easier and more pleasant than using documentation that is composed of massive amounts of text and configuration printouts. Network configuration tables should hold essential information about the network devices and not be cluttered with unimportant...
Commands and Applications Used to Correct Problems Occurring at the Physical and Data Link Layers
The CIT course discusses certain commands with regards to correcting physical and data link problems. This section discusses those commands. Neither the CIT course nor this book imply by any means that you can fix all physical and data link layer problems by using these commands. The commands are listed and briefly described in Table 8-2. From there, a more detailed discussion of these controller and interface configuration commands is provided. The last part of this section gives examples on...
Resolving Problems at the Physical or Data Link Layer
Chapter 7, Isolating a Problem at the Physical or Data Link Layer, lists several common symptoms of problems occurring at the physical or data link layer. Table 13-1 provides a summary of those symptoms for your review. Table 13-1 Common Symptoms of Physical and Data Link Layer Problems Table 13-1 Common Symptoms of Physical and Data Link Layer Problems Common Symptoms of Problems Occurring at the Physical Layer Common Symptoms of Problems Occurring at the Data Link Layer No component above the...
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800 East 96th Street, 3rd Floor Indianapolis, Indiana 46240 USA CCNP CIT Exam Certification Guide Second Edition Amir S. Ranjbar, MSc., CCIE No. 8669 Copyright 2004 Cisco Systems, Inc. 800 East 96th Street, 3rd Floor Indianapolis, IN 46240 USA All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without written permission from the...
The Process of Creating Network Documentation
To create network documentation, you must log into each device, one at a time, and discover and document its network configurations. You can hop from one device to its undocumented neighbor(s) and repeat this process until you are finished. Figure 1-2 depicts the stage-by-stage process of creating network documentation by using a flow chart. Table 1-6 provides an explanation for each of the stages shown in Figure 1-2. Figure 1-2 Creating Network Documentation Figure 1-2 Creating Network...
Analyzing Commands and Applications Used to Isolate Problems Occurring at the Physical and Data Link Layers
This section covers commands that you use to isolate problems at the physical and data link layers. This section also demonstrates isolating problems at physical and data link layers through some examples. Table 7-2 shows end system commands used to isolate problems at the physical and data link layers. The first part of this table lists general commands. The second section lists commands you enter at an end system running a version of the Windows operating system. In the third part of Table...
Comparing Layered Networking Models
This section explains the 4-layer TCP IP model by comparing it to the OSI model and by describing the function of each of its layers. Figure 3-2 displays the 7-layer OSI model and the 4-layer TCP IP model side by side. As displayed, similar to the OSI networking model, the TCP IP networking model divides networking architecture into modular layers. Figure 3-2 shows how the TCP IP networking model maps to the layers of the OSI networking model. Figure 3-2 Comparing the OSI Model with the TCP IP...
The BottomUp Troubleshooting Approach
The bottom-up approach to troubleshooting a networking problem starts with the physical components of the network and works its way up the layers of the OSI model. If you conclude that all the elements associated to a particular layer are in good working condition, you inspect the elements associated with the next layer up until the cause(s) of the problem is are identified. Figure 6-1 shows the bottom-up troubleshooting approach. Figure 6-1 A Bottom-Up Troubleshooting Approach Figure 6-1 A...
Identifying the Symptoms of Problems Occurring at the Data Link Layer
When the data received from or sent to the network layer is not properly encapsulated into or de-encapsulated from data link layer frames, or when data link layer frames are not properly formed, transmitted, and received across working physical layer links, you have data link layer problem(s). Improper frame types (mismatched encapsulations), duplicate MAC addresses, and misbehaving Layer 2 devices such as switches or bridges (due to various configuration or hardware problems or limitations...
Commands and Applications Used to Isolate Problems Occurring at the Application Layer
This section presents a set of commands that you can use to isolate problems at the application layer. First, it introduces a set of commands used on end systems. Next, it discusses a set of Cisco IOS commands that display information related to host table, e-mail (Post Office Protocol 3 POP3 , SMTP, Internet Message Access Protocol IMAP ), network management (Simple Network Management Protocol SNMP , NTP), file management (FTP, TFTP), Telnet, and DHCP. The ifconfig -a command is a useful...
Commands and Applications Used to Isolate Problems Occurring at the Transport Layer
To ascertain that a problem is rooted at the transport layer rather than at a layer below it, you must use proper tools and commands. You can test the operability of Layers 1, 2, and 3 by using the Windows commands ping, tracert, netstat, and nbtstat. If the results prove that the lower layers (physical, data link, and network) are in good working order, you can focus your attention on Layer 4-related issues such as filters, policies, quality of service (QoS) tools, and so on. You can use the...
Analyzing Cisco Command and Application Output to Isolate Problems Occurring at the Network Layer
This section describes several important Cisco IOS commands that display information that is useful for isolating network layer problems (see Table 9-3). Following that, a scenario-style example demonstrates how you can use the commands listed in Table 9-3 to help isolate a realistic network layer problem. The first few commands listed in Table 9-3 are marked as general commands because they might reveal issues related to many aspects of configuration at the network layer (addressing, routing,...
Resolving Problems at the Network Layer
You learned in Chapter 9, Isolating a Problem at the Network Layer, that common and possible symptoms of network layer problems include the following On the failing link, no component appears to be functional above the network layer. The network is operating, but it is operating either consistently or intermittently below the baseline level. The transport layer cannot transfer data or achieve connectivity. Routing tables are empty, inconsistent, or incomplete. Routing behavior is unexpected....
Symptoms of Problems Occurring at the Application Layer
The application layer of the TCP IP suite maps to the session (Layer 5), presentation (Layer 6), and application (Layer 7) layers of the OSI model. Therefore, issues that are related to any of these layers should be referred to as application layer problems within the TCP IP framework. You can categorize a problem as a pure application layer problem if you can prove that all the lower layers up to the transport layer are in good working condition. Application layer problems might be rooted at...
The Divideand Conquer Troubleshooting Approach
The divide-and-conquer approach to network troubleshooting, unlike its top-down and bottom-up counterparts, does not always commence its investigation at a particular OSI layer. When you apply the divide-and-conquer approach, you select a layer and test its health based on the observed results, you might go in either direction (up or down) from the starting layer. Figure 6-3 depicts the divide-and-conquer troubleshooting approach. If a layer is in good working condition, you inspect the layer...
Dshow ip interface brief
You have been informed that a group of users lost its connection to the network due to a problem in either the physical or data link layer. You check the operational status, errors, and configuration of the affected interfaces. You still have not found the cause of the problem. Which guideline for isolating a problem at the physical and data link layers have you omitted a. Verifying the IP address of the default gateway b. Checking cable configuration d. Viewing TCP information on a host 5....
The Symptoms of Problems Occurring at the Network Layer
When the network layer or higher experiences symptoms or difficulties, it might be difficult to determine exactly at which layer the root problem is located. The reason is that the symptom or problem might also have sprung from physical or data link layer failures. If troubleshooting efforts show that the physical and data link layers are problem free, then you have a concrete indication that your problem resides at the network layer or higher. Following are some common symptoms of network...
Components of an End System Network Configuration Table
The purpose of this section is to identify the information that you should record in an end system network configuration table used to troubleshoot end systems. The network configuration table, as baseline documentation, should hold accurate and up-to-date information about the software and hardware used in end systems. The following is a list of items to record in an end system network configuration table Device name and its role or function (purpose), if any Default gateway, DNS server, and...
Network Documentation
In Chapter 1, Creating Network Configuration Documentation, you learned that to create effective network documentation, you must do the following Determine the scope of your work You must find out what part of the network you are responsible for and which devices you need to include in your work. Know your objective Collect and document information about the elements within your scope. While making certain that you provide sufficient detail, you must also avoid collecting and documenting too...
The Encapsulated Data Flow Process
This section discusses the stages of data flow through a network that interconnects remote end systems and segments using Cisco routers and switches. This discussion includes the topic of encapsulation and decapsulation (or de-encapsulation), along with the different forwarding filtering performed by routers and switches. Figure 3-1 shows end system Host A and end system Host B communicating on a network. The encapsulated data flow process has four stages. In Figure 3-1, Stages 1 through 4...
The Top Down Troubleshooting Approach
As its name implies, when you apply a top-down approach to troubleshooting a networking problem, you start with the user application and work your way down the layers of the OSI model. Figure 6-2 shows the top-down troubleshooting approach. If a layer is not in good working condition, you inspect the layer below it. When you know that the current layer is not in working condition and you discover that a lower layer works, you can conclude that the problem is within the layer above the lower...
Gathering Network Symptoms
Figure 5-1 displays the process of gathering symptoms from a network using a flowchart. Figure 5-1 sends the message that you must first ascertain that the problem is within your area of responsibility. Next, you must determine whether the symptoms and facts gathered are enough to identify the cause and take corrective actions. Sometimes gathering more facts is necessary when you have gathered enough information and determined the cause, you can take corrective actions. The stages of gathering...
Gathering Symptoms
Chapter 5, Gathering Symptoms, presents a flow diagram (see Figure 13-1) that depicts the process of gathering symptoms from a network. Figure 13-1 Gathering Network Symptoms Figure 13-1 Gathering Network Symptoms The process of gathering symptoms from a network is composed of the following steps 1. Analyze Existing Symptoms After you gather symptoms from the users, end systems, or trouble ticket documents, you must analyze the symptoms and describe the problem. 2. Determine Ownership Based on...
Components of a Topology Diagram
The network topology diagram is the second piece of documentation (after the network configuration table), and it is considered an essential part of any network baseline. This diagram is a graphical representation of the network that must illustrate all the devices and how they are connected. Physical and logical detail about the network are revealed using consistent notations and symbols. Figure 1-1 is an example of a network topology diagram. In Figure 1-1, you can see a network cloud symbol...
Identifying the Symptoms of Problems Occurring at the Physical Layer
Because the components of the OSI model's physical layer are tangible and can be inspected physically, this layer is somewhat unique relative to the other layers. Examples of the physical layer components include interfaces ports, modules, cables, connectors, adapters, transceivers, and so on. Problems at the physical layer might cause complete or intermittent loss of data across a link, generating symptoms such as application failures or data transfers at lower than expected rates. If you log...
End System Commands and Applications Used to Isolate Problems Occurring at the Network Layer
Table 9-2 describes some general end system commands, some commands specific to Microsoft Windows operating systems, and some UNIX Mac OS X commands that you can use to isolate problems at the network layer. Although many such commands also display information that concerns the data link layer, they are noteworthy at the network layer because they highlight problems in the interface between the data link and network layers. Table 9-2 End System Commands to Isolate Network Layer Problems Table...
The General Troubleshooting Process
Solving network problems effectively and in a timely manner requires following a systematic troubleshooting methodology. The goal of this chapter is to present a general process and methodology that you can use in any network troubleshooting situation. The benefit of applying this general process is that it helps you systematically resolve problems quicker and more cost effectively, with less confusion and wasted time. To appreciate the importance of using a systematic troubleshooting method,...
CIT Exam Topics
Cisco lists the topics for the CIT exam on its Web site at http www.cisco.com warp public 10 The list provides key information about what the test covers. Table I-1 lists the CIT exam topics and the corresponding parts in this book that cover those topics. Each part begins with a list of the topics covered in that part. Use these references as a road map to find the exact materials you need to study to master the CIT exam topics. Note, however, that because all exam information is managed by...