As a Network
Engineer, your primary goal is to make sure that your network equipment is
operating properly at all times. But we all know that any equipment can break
down. The reasons for this could be software inconsistencies, hardware
malfunctions, maybe even environmental hazards.
Under such
unpredictable conditions, your responsibility is to identify and isolate the
cause of the malfunction and correct it as soon as you can. That’s why it is
extremely helpful to know some specific techniques that have been proven to be
crucial and essential in the networking world.
In today’s article
I’ll present the most important commands that you will definitely find helpful
and even mandatory throughout your networking career and specifically during
network troubleshooting situations. The commands I am talking about, and which
are truly invaluable, are:
- Ping
- Traceroute
- Telnet
- Show interfaces
- Show ip interface
- Show ip route
- Show running-config
- Show startup-config
The Ping
Utility
The PING
command operates on the Network layer and uses the services of the
ICMP protocol. It is the first command that you should use at the beginning of
your troubleshooting process.
With PING you can
test whether a remote host is alive by transmitting echo request messages and
receive echo replies from the specific host. Keep in mind that even if a host
is alive, it does not mean that it is functioning properly that is why PING is
used at the beginning of your investigation and is the best command to start
with.
PING command has
quite a lot of options from which you can greatly benefit. For example, you can
choose to PING with different network protocols. Moreover, you are able to set
the exact number of PING requests to be transmitted. You can also choose the
length of data carried within the echo request packet (provided in bytes) and
also specify whether fragmentation of this packet is allowed or not during
transmission. The last two options in cooperation can be used to identify the
lowest MTU value existing in the communication path.
On Cisco devices
the simpler way to use the PING utility is to issue the command PING along with
the IP address of the remote device:
For more advanced
features, simply issue the PING command and follow the prompts from that point
on:
The Traceroute
Command
The TRACEROUTE
command traces the end-to-end path a packet takes though an
internetwork. Similarly with PING, it uses the ICMP protocol with TTL timeouts
to perform its operation..
This command is
very useful in identifying potential link bottlenecks throughout the
transmission path. Here is a sample output of the TRACEROUTE command:
The Telnet
Command
Use the telnet
command to verify TCP stack and application layer software between
source and destination stations. Of course, to be able to telnet on a Cisco
device, the latter needs to be already configured to accept telnet connections.
To use TELNET just issue the telnet command along with the IP address or
hostname of the remote station:
The Show
Interfaces Command
The show
interfaces command presents all the available interfaces that can be
configured on your Cisco device. You can explicitly use this command to show
only details on a single interface by issuing the interfaces name after the
show interfaces command. This command is very useful because it can reveal
layer 1 and layer 2 problems. Moreover, this command provides details regarding
the hardware address (MAC), IP address, encapsulation method and statistics
concerning erroneous conditions on the specific interface. Examine the output
of the show interface fastethernet 0:
The first line of
the show interface command is the most important one. The first parameter
refers to the physical layer, thereforein this case FastEthernet 0 is
up and operating. The second part of the line refers to the Data Link Layer;
therefore here line protocol is up means that we have layer 2 connectivity as
well. There are four possible outputs that you may come across:
- FastEthernet0 is up, line protocol is up: Both the Physical and Data Link layers on the interface are functioning correctly.
- FastEthernet0 is down, line protocol is down: This output indicates a physical interface problem. For example, the cable on this interface or on the remote interface is disconnected.
- FastEthernet0 is up, line protocol is down: In this case, Physical layer is operational. The line protocol being down indicates a clocking or framing problem. Probable reasons for this are encapsulation and clock rate mismatches.
- Ethernet0 is administratively down, line protocol is down: This output indicates that a local interface has been manually shut down using the shutdown command.
The Show IP
Interface Command
The show
ip interface command will provide details regarding layer 3
configuration on the interfaces. Using this command you can see the IP address
and mask configured on a given interface, whether an access list is applied on
the interface as well as basic
IP information.
IP information.
The Show IP
Route Command
Use the show
ip route command to find detailed information regarding the routes
configured on the router. Keep in mind that the router can only route packets
to the networks listed in its routing table. It is possible that a router
cannot reach a network you manually configured therefore, that specific route
is removed from its table and that is why you should use the show running
configuration command in conjunction with show ip route to spot possible
inconsistencies.
Show
Running-Config and Show Startup-Config Commands
Issue the show
running configuration command to find out the whole configuration your
Cisco devices use while operating. By looking at the details of your devices,
configuration can help you identify the cause to your problem. Use the show
startup configuration command to see the configuration commands that
will be loaded to your device the next time it reboots and compare it with your
running configuration in order to identify possible unconformities.
In the End … Follow Your Instincts
Isolating a
network malfunction is not an easy task. In many cases you need to swim into
really deep waters in order to identify the error. Meaning, that you will
probably need to get into the bits and bytes in order to locate the error.
A lot of network
monitor tools are available to help you during these difficult tasks. And
although there’s a lot you can do, remember to never lose your courage; have
patience and follow your gut.
Sometimes the
problem that you’re searching for will be right in front of you and will slip
out of your hands, just when you think you have it. So be persistent, be
patient and remember: you will win the battle!
Be prepared; in
order to fight well, you should be armed well. So get to know your device very
well, know how it operates, and know how to use the basic troubleshooting
commands – that you just learned – so that you can identify the problem, or at
least estimate where the problem might be. Good luck!
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