CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 83 doc

Layer 1 physical layer errors can include the following: ■ Broken cables ■ Disconnected cables ■ Cables connected to the wrong ports ■ Intermittent cable connection ■ Wrong cables used f

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Introduction to Network Testing 789

Figure 18-3 shows one approach to troubleshooting network problems

Figure 18-3 OSI Layer Troubleshooting

With a structured approach, members of the network know what each member has

completed to solve a problem If a variety of ideas are used without any organization,

the problem solving becomes chaotic Without a structured approach, very few

prob-lems are solved

Start

Layer 1 OK?

Yes

Layer 2 OK?

Yes

Layer 3 OK?

Yes

Layer 4 OK?

Yes

Layer 5 OK?

Yes

Layer 6 OK?

Yes

Layer 7 OK?

Yes

All OK?

Yes

No Start

Stop

chpt_18.fm Page 789 Tuesday, May 27, 2003 2:19 PM

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790 Chapter 18: Basic Router Troubleshooting

The ﬂowcharts in Figure 18-2 and Figure 18-3 are not the only methods of trouble-shooting; however, the orderly process is very important to ensure that a network runs smoothly and efﬁciently

Testing by OSI Layers

Testing should begin with Layer 1 of the OSI model and work to Layer 7, if necessary Layer 1 (physical layer) errors can include the following:

■ Broken cables

■ Disconnected cables

■ Cables connected to the wrong ports

■ Intermittent cable connection

■ Wrong cables used for the task at hand (must use rollovers, crossover cables, and straight-through cables correctly)

■ Transceiver problems

■ Data communications equipment (DCE) cable problems

■ Data terminal equipment (DTE) cable problems

■ Devices turned off After exhausting all Layer 1 problems as possibilities for a network problem, the next step is to investigate Layer 2 problem possibilities

Layer 2 errors can include the following:

■ Improperly conﬁgured serial interfaces

■ Improperly conﬁgured Ethernet interfaces

■ Improper encapsulation set (High-Level Data Link Control [HDLC] is default for serial interfaces)

■ Improper clock rate settings on serial interfaces

■ Network interface card (NIC) problems After exhausting all Layer 2 problems as possibilities for a network problem, the next step is to investigate Layer 3 problem possibilities

Layer 3 errors can include the following:

■ Routing protocol not enabled

■ Wrong routing protocol enabled

■ Routing protocol incorrectly conﬁgured

■ Incorrect IP addresses

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■ Incorrect subnet masks

■ Incorrect default gateway

If errors or problems related to connectivity appear on the network, testing through

the layers using connectivity tools, such as ping or telnet, should be the ﬁrst step in

determining the point in the network where the packet gets dropped The ping

com-mand can be used at Layer 3 to test connectivity At Layer 7, the telnet comcom-mand can

be used to verify the application layer software between source and destination stations

Both of these commands are discussed in detail later in the chapter

Troubleshooting Layer 1 Using Indicators

Indicators are useful tools for troubleshooting Most interfaces or NICs have indicator

lights that show whether there is a valid connection This light usually is referred to as

the link light The interface also might have lights to indicate whether trafﬁc is being

sent (TX) or received (RX) If the interface has indicator lights that do not show a

valid connection, power off the device and reset the interface card The NIC functions

at both Layers 1 and 2

Many indicators are considered Layer 1 problems in a network, including the following:

■ Broken cables

■ Disconnected cables

■ Cables connected to the wrong ports

■ Intermittent cable connection

■ Wrong cables used for the task at hand

■ Transceiver problems

■ Devices turned off

A faulty or incorrect cable could result in a link light indicating a bad connection or

no link

Check to make sure that all cables are connected to the appropriate ports Make sure

that all cross-connects are patched properly to the correct location using the

appropri-ate cable and method Verify that all switch or hub ports are set in the correct VLAN

or collision domain, and have appropriate options set for Spanning Tree and other

considerations

Verify that the proper cable is being used If a direct connection exists between two

end systems—for example, between a PC and a router or between two switches—a

special crossover cable might be required Verify that the cable from the source interface

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is connected properly and is in good condition If it does not seem to be a good con-nection, reset the cable and ensure that the connection is secure Try replacing the cable with a known working cable If the cable connects to a wall jack, use a cable tester to ensure that the jack is wired properly

Also check any transceiver in use to ensure that it is the correct type and is properly connected and conﬁgured If replacing the cable does not resolve the problem, try replacing the transceiver, if one is being used

Before you run diagnostics or attempt complex troubleshooting, always check to make sure that the device is powered on Some problems result from the simplest of errors

Troubleshooting Layer 2 Using the show interface Command

The show interface command is perhaps the single most important tool to discover

Layer 1 and Layer 2 problems with the router The ﬁrst output parameter, or line, refers

to the physical layer The second parameter, or protocol, indicates whether Cisco IOS Software processes that control the line protocol consider the interface usable This is determined by whether keepalives successfully are received If the interface misses three consecutive keepalives, the line protocol is marked as down

If the line is down, the protocol also must be down This is because there would be no usable media for the protocol This occurs when the interface is down because of a hardware issue or if it is “administratively down” because of a conﬁguration issue

If the interface is up and the line protocol is down, a Layer 2 problem exists The following list contains some possible causes:

■ No keepalives

■ No clock rate

■ Mismatch in encapsulation type

These problems can be determined when the show interface command has been

entered and a line appears stating, “interface is up and the line protocol is down.”

Using the clockrate command or the encapsulation command, verify that that there is

no mismatch between interfaces

The show interfaces serial command should be used after conﬁguring a serial interface,

to verify the changes and ensure that the interface is operational

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Troubleshooting Layer 3 Using ping

Thepingutility is used to test network connectivity To aid in diagnosing basic

net-work connectivity, many netnet-work protocols support an echo protocol, which is used

to test whether protocol packets are being routed The ping command sends a packet to

the destination host and then waits for a reply packet from that host Results from this

echo protocol can help evaluate the path-to-host reliability, delays over the path, and

whether the host can be reached or is functioning The ping output displays the

mini-mum, average, and maximum times that it takes for a ping packet to ﬁnd a speciﬁed

system and return ping uses the Internet Control Message Protocol (ICMP) to verify

the hardware connection and the logical address of the network layer This is a basic

testing mechanism

For the network in Figure 18-4, the ping target 172.16.1.5 responded successfully to

all ﬁve datagrams sent, as shown in Example 18-1

Figure 18-4 Network for Testing with ping

The exclamation points (!) indicate each successful echo If any periods (.) instead of

exclamations are received on a display, the application on the router timed out waiting

for a given packet echo from the ping target The ping command can be used to

diag-nose basic network connectivity

Example 18-1 ping Results for Target 172.16.1.5

Router> ping 172.16.1.5

Type escape sequence to abort

Sending 5, 100 byte ICMP Echos to 172.16.1.5,

timeout is 2 seconds:

!!!!!

Success rate is 100 percent,

round-trip min/avg/max – 1/3/4 ms

Router>

Network Layer

Echo Request Echo Reply

172.16.1.5

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ping tests network connections by sending ICMP echo requests to a target host and waiting for replies When testing a connection, ping tracks the number of packets sent, the number of replies received, the percentage of packets lost, and the amount of time required for the packets to reach the destination and for replies to be received This information enables users to verify whether their workstations can communicate with other hosts, and they can determine whether any information was lost

The ping command can be initiated from both user EXEC mode and privileged EXEC mode The ping command can be used to conﬁrm basic network connectivity on

AppleTalk, ISO Connectionless Network Service (CLNS), IP, Novell, Apollo, VINES, DECnet, or XNS networks

ICMP uses messages to accomplish various tasks Table 18-1 shows a list of ICMP message types

Table 18-1 CMP Message Types

Destination unreachable This tells the source host that there is a

prob-lem delivering a packet

Time exceeded The time that it takes a packet to be delivered

has been too long, and the packet has been discarded

Source quench The source is sending data faster than it can

be forwarded This message requests that the sender slow down

Redirect The router sending this message has received

some packet for which another router would have had a better route The message tells the sender to use the better route

connectivity

Parameter problem This is used to identify a parameter that is

incorrect

Timestamp This is used to measure round-trip time to

particular hosts

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The extended command mode of the ping command permits users to specify the

sup-ported IP header options This allows the router to perform a more extensive range of

test options To enter ping extended command mode, type the ping command with no

options in privileged mode and press Enter You are prompted for some basic options

as well as the Extended Commands option Enter yes at the Extended commands prompt

to specify various extended ping options An extended ping works the same way as a

regular ping, but it supports the manipulation of some different parameters, such as

packet size and frequency

It is a good idea to use the ping command when the network is functioning properly, to

see how the command works under normal conditions and so that you have something

to compare against when troubleshooting

Troubleshooting Layer 3 Issues Using traceroute

Thetraceroutecommand is used to discover the routes that packets take when traveling

to their destinations traceroute also can be used to test the network layer, or Layer 3,

on a hop-by-hop basis and to provide performance benchmarks

The output of the traceroute command generates a list of hops that successfully were

reached, as shown in Example 18-2 If the data successfully reaches the intended

desti-nation, then the output indicates every router that the datagram passes through This

output can be captured and used for future troubleshooting of the internetwork

Address mask request/reply This is used to inquire about and learn the

correct subnet mask to be used

Router advertisement and selection This is used to allow hosts to dynamically

learn the IP addresses of the routers attached

to the subnet

Example 18-2 traceroute Command Output

Cougars> traceroute 168.71.8.2

traceroute to pc-b.cisco.com (168.71.8.2), 30 hops max, 40 byte packets

1 routerb (168.71.6.3) 3 ms 3 ms 3 ms

2 * * *

continues

Table 18-1 CMP Message Types (Continued)

3 ms 3 ms 3 ms

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traceroute output also indicates the speciﬁc hop at which the failure is occurring For each router in the path, a line of output is generated on the terminal indicating the IP address of the interface that the data entered If an asterisk (*) appears, the packet failed You can isolate the problem area by obtaining the last good hop from the trace-route output and comparing it to a diagram of the internetwork

traceroute also provides information indicating the relative performance of links The round-trip time (RTT) is the time required to send an echo packet and get a response,

as shown in Example 18-2 In this example, each of the three packets sent has an RTT

of 3 milliseconds (ms) This is useful for an approximate idea of the delay on the link These ﬁgures are not precise enough to be used for an accurate performance evaluation; however, this output can be captured and used for future performance troubleshooting

of the internetwork

The device receiving the traceroute also must know how to send the reply back to the source of the traceroute For the traceroute or ping data to successfully make the round trips between routers, there must be known routes in both directions A failed response is not always an indication of a problem because ICMP messages could be rate-limited or filtered at the host site This is especially true across the Internet traceroute sends out a sequence of User Datagram Protocol (UDP) datagrams from the router to an invalid port address on the remote host For the first sequence of three datagrams sent, a Time-To-Live (TTL) field value is set to 1 The TTL value of 1 causes the datagram to time out at the first router in the path This router then responds with

an ICMP time exceeded message (TEM) indicating that the datagram has expired Three more UDP messages now are sent, each with the TTL value set to 2 This causes the second router to return ICMP TEMs This process continues until the packets reach the other destination

Because these datagrams are trying to access an invalid port at the destination host, ICMP port unreachable messages are returned instead of ICMP TEMs This indicates

an unreachable port and signals the traceroute program, terminating the process

3 * * *

4 * * *

5 * * * Cougars>

Example 18-2 traceroute Command Output (Continued)

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Troubleshooting Layer 7 Using Telnet

Telnet is a virtual terminal protocol that is part of the TCP/IP protocol suite Telnet

allows the veriﬁcation of the application layer software between source and destination

stations This is the most complete test mechanism available Telnet allows connections

to be made to remote devices, to gather information and run commands and applications

Telnet is the most complete testing tool because it uses IP at the network layer and

TCP at the transport layer to create a session with a remote host If Telnet is successful,

IP connectivity must be good

The telnet command provides a virtual terminal so that administrators can use Telnet

operations to connect with other network devices (such as routers and switches)

run-ning TCP/IP If a router can be remotely accessed using Telnet, at least one TCP/IP

application can reach the remote router A successful Telnet connection indicates that

the upper-layer application and the services of lower layers are functioning properly

Figure 18-5 illustrates a Telnet connection

Figure 18-5 Using Telnet

If an administrator can Telnet to one router but not to another router, the Telnet

fail-ure likely is caused by speciﬁc addressing, naming, or access permission problems These

problems can exist on the administrator’s router or on the router that failed as a Telnet

target

If the Telnet to a particular server fails from one host, try connecting from a router and

several other devices Also try using ping as a more basic test If you can ping the host,

but cannot achieve a login prompt when trying to Telnet, check the following:

■ Can a reverse DNS lookup on the client’s address be found? Many Telnet servers

do not allow connections from IP addresses that have no DNS entry This is a common problem for DHCP-assigned addresses in which the administrator has not added DNS entries for the DHCP pools

Application

Telnet

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■ A Telnet application might not be capable of negotiating the appropriate options and, therefore, will not connect On a Cisco router, this negotiation process can

be viewed using debug telnet Look for error messages or invalid IP or DNS

address, which would indicate this problem

■ It is possible that Telnet is disabled or has been moved to a port other than 23 on the destination server Recall that port 23 is the default port for Telnet

Troubleshooting Router Issues Using the show interface and show interfaces Commands

Cisco IOS Software contains numerous commands for troubleshooting Among the

more widely used are the show commands Every aspect of the router can be viewed with one or more of the show commands The show interface command is used to

check the status and statistics of the interfaces Variations of this command can be used to check the status of the different types of interfaces To view the status of the

FastEthernet interfaces, use show interfaces fastethernet This command also can be

used to view the status of one particular interface To view the status of the Serial 0/0

interface, use show interface serial0/0 To view the status of FastEthernet 0/0, use show interface fa0/0.

The show interfaces command displays the status of two important portions of the

interfaces These can be related to the Layer 1 and the Layer 2 functions:

■ Physical (hardware) portion—The hardware includes cables, connectors, and

interfaces showing the condition of the physical connection between the devices

■ Logical (software) portion—The software status shows the state of messages

such as keepalives, control information, and user information that are passed between adjacent devices This relates to the condition of a data link layer protocol passed between two connected neighboring router interfaces

These important elements of the show interface serial command output are displayed

as the line and data-link protocol status shown in Example 18-3

Lab Activity Troubleshooting Using ping and telnet

In this lab, you use knowledge of OSI Layers 1, 2, and 3 to diagnose network

conﬁguration errors while using the ping and telnet utilities.

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