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
- Failure to perform optimally
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: - Has anything in the network
changed recently?
- Is there anyone currently working
on the network infrastructure?
Connectivity
Issues:
Check for any equipment and connectivity
problems including: - Power problems: outages,
intermittent problems, environmental problems such as
overheating, and so on.
- Layer 1 problems: cabling
problems, bad ports, service provider problems, and so on.
Some commands, which can be used to check these and
other issues, include: - ping
- traceroute
- show interface, show ip
interface
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