Content Overview Network layer problems include any problem which involves a Layer 3 protocol, both routed protocols and routing protocols. This module will focus primarily on IP routing protocols. Issues concerning routed protocols, such as IP, and other Layer 3 IP protocols, are discussed in the other modules. Network layer problems may also include issues at the other layers. Configuring routing over a nonbroadcast multiaccess (NBMA) or dialup network can involve specific configuration and troubleshooting issues. Because these configurations involve a specific Layer 2 technology such as Frame Relay, ISDN, and so on, routing problems involving these technologies will be discussed in the module Troubleshooting Layer 2. This module will focus on the most common troubleshooting issues regarding both static routing and dynamic routing protocols. Troubleshooting dynamic routing protocols is separated by common issues pertaining to most routing protocols, such as Layer 1 issues, incorrect network statements, and by routing protocols including RIP, IGRP, EIGRP, OSPF, IS-IS, and BGP. Troubleshooting redistribution issues with routing protocols are also discussed in this module.
Content 5.1 Troubleshooting Network Layer Problems 5.1.1 What are network layer problems? Network layer problems include any problem that involves a Layer 3 protocol, including both routed and routing protocols. This module will focus primarily on IP routing protocols. Problems at the network layer can have the following affects on a network: Network failure is when the network is nearly or completely nonfunctional, affecting all users and applications using the network. These failures are usually noticed quickly by users and network administrators, and are obviously critical to the productivity of a company. Network optimization problems can be more difficult to discover and sometimes harder to diagnose. These problems usually involve a subset of users, applications, destinations, or a particular type of traffic. Optimization issues in general can be more difficult to detect and even harder to isolate and diagnose as they usually involve multiple layers or even the host computer itself. Determining that the problem is a network layer problem can take time. Aspects of network optimization are discussed in more detail in the module “Layer 1–7 Troubleshooting”. A possible optimization problem on a link at the network layer can be identified because data will be transferred in a slow, sporadic, or unexpected manner compared to the data transfer rate documented in the baseline. Symptoms short of a complete failure to route packets may include misrouted packets that are lost or experience excessive transit delays. Many of the problems discussed in this module can apply to both network failures and optimization problems, depending upon the scenario and configuration of the actual network. Not all of the information in this module has to do with troubleshooting routing problems. This module also contains information regarding routing optimization, which includes some additional information on how the routing table process operates. Understanding the routing table process is helpful when troubleshooting certain situations, so that the cause of the problem is understood, not just how to solve it. Lab Activity Lab Exercise: Isolating Network Layer Problems After completing this lab, the student will be able to analyze user feedback and end-system data to decide in which OSI layer to begin isolating problems.
Content 5.1 Troubleshooting Network Layer Problems 5.1.2 Isolating the problem methodology Although this module will describe many common problems and solutions for troubleshooting various routing protocols, it is important to note that there is no single template for solving these Layer 3 problems. Routing problems are solved with a good methodical process, using a series of commands to isolate and diagnose the problem. Many of the examples in this module are just that, examples. It is important to be able to apply these examples and concepts to other routing protocols and other scenarios. The first step is to isolate the problem. This can be done by identifying and documenting the symptoms. Here are some areas to explore when diagnosing a possible problem involving routing protocols: General Network Issues:
Many times a change in the topology, such as a down link, may have other affects on other areas of the network which might not be obvious at the time. This may include the installation of new routes, static or dynamic, removal of other routes, and so on. Some of the things to consider include: Connectivity Issues:
Check for any equipment and connectivity problems including: Some commands, which can be used to check these and other issues, include: Neighbor issues – If the routing protocol establishes an adjacency with a neighbor, check to see if there are any problems with the routers forming neighbor relationships. Topology database – If the routing protocol uses a topology table or database, check the table for anything unexpected, such as missing entries or unexpected entries. Routing table – Check the routing table for anything unexpected, such as, missing routes or unexpected routes. Use debug commands to view routing updates and routing table maintenance. Many troubleshooting issues and solutions are common amongst different routing protocols. At times, this could be due to whether the routing protocol is a distance-vector or link state routing protocol, if the routing protocol uses AS or process-id numbers, whether or not the routing protocol forms neighbor relationships, is the routing protocol classful or classless, and whether or not the routing protocol has a topology database. For example, split horizon issues will be common with all IGP distance-vector routing protocols, RIP, IGRP, and EIGRP. With IGRP and EIGRP, their AS numbers must be the same for routers to exchange updates, whereas with OSPF the process-id number is local only to that router. In this module, some issues described on one routing protocol may also apply to others. Lab Activity Lab Exercise: Correcting Problems at the Network Layer In this lab, the student will use various Cisco commands to correct network problems.
Content 5.1 Troubleshooting Network Layer Problems 5.1.3 Static routes, dynamic routing, summarization, redistribution, and combinations Most networks are not configured with only static routes or with only one type of dynamic routing protocol. Most networks are a complex combination of static routes, one or more dynamic routing protocols, manual and automatic route aggregations, redistributions, distribute-lists, access lists, and more. Understanding how these protocols and techniques work together and what affect they can have on each other is essential in understanding and troubleshooting networks. The course examines these issues separately and, at times, in combination. It is impossible to cover every possible scenario and combination. There are entire books on each one of these topics, so there is no way to cover it all in this module or this course. What is important is to have a thorough understanding of the routing protocols and configurations used in many networks. The more time spent now understanding how the routing protocols function and how they interoperate with other routing protocols, static routes, and the routing table, the less time will be spent later trying to