ip-address} command is used to verify
connectivity between hosts by sending an ICMP echo request to
the target IP address. If all host unreachable results are
returned to the sending station, this indicates that the
station does not have connectivity to the network. It is
typical to ping the directly attached router interface of a LAN
segment, then ping progressively further into the network to
establish the point at which loss of connectivity occurs. The
Arp –a is a very useful interrogatory command because it
establishes whether there is Layer 2 and Layer 3 connectivity
within a LAN segment. The ARP table is flushed and renewed
periodically. If this command does not return a list of MAC
addresses and associated IP addresses then the loss of
connectivity is very likely to be at the physical layer. This
would also be the case if the MAC address corresponding to the
attached router interface was not present. While peer machines
are still present in an ARP table the link to the router
interface or the interface itself is likely to be the problem.
Content 3.3 Windows and Cisco
Commands for Physical Layer Information Gathering
3.3.2 End-system commands – Windows only For
Windows end systems, the user often queries a network
connection at the MS-DOS command prompt. By entering commands
at the DOS command prompt, or by using the Run window in the
Start menu, a user can establish whether a connection is
present with other network devices. Also the path that data
takes to reach the destination can be determined, which can be
used to verify multiple links and also the reliability of any
connection. The ipconfig /all command is a simple way
to check connectivity on a Windows NT/2000/XP system. It will
identify the host MAC address, DNS, DHCP, NT and WINS servers
that the host is attached to if it has physical connectivity.
The tracert command can be used to test connectivity to
a destination device. It will enable a user to map the route
taken on the way to the destination. The winipcfg
command is used in older version of 9x Windows up to Me. As the
name suggests winipcfg will show the Windows IP
configuration information.
Content
3.3 Windows and Cisco Commands for
Physical Layer Information Gathering 3.3.3
End-system commands – UNIX/Mac OS For UNIX or Mac OS
end-systems the user often queries the status of the network
connection by using run-mode to execute network connectivity
queries. By entering commands a user can establish whether a
connection is present with other network devices, the path that
data takes to reach the target and also the reliability of any
connection. The ifconfig –a command performs the same
function that the ipconfig command performs for Windows
NT/2000/XP systems. It will list the IP address information for
a Mac OS X and UNIX hosts. The traceroute command can
be used to show the path a packet takes through the network. It
is useful to identify at what point a link is broken, or
sub-optimal, in the network.
Content
3.3 Windows and Cisco Commands for
Physical Layer Information Gathering 3.3.4
Common Cisco IOS commands A variety of commands exist under
the Cisco IOS to gather information on connectivity problems.
When a router experiences an absence or intermittent loss of
connectivity to other network components, the device can be
interrogated for statistics collected on a port or interface.
Additionally networking processes can be monitored in real-time
to determine at what point in the sequence connectivity is
lost. In the case where numerous routes exist to a desired
destination, which is suffering intermittent or total loss of
connectivity, information on the actual point at which the
break occurs can be collected.
Content
3.3 Windows and Cisco Commands for
Physical Layer Information Gathering 3.3.5
Cisco IOS show commands Show commands represent the most
complete set of tools in the IOS armory for collecting
information on network outages. A very wide range of parameters
can be examined individually, or in combination with other
related parameters. Individual components in a router or switch
can be examined, from a number of different angles. Use the
show commands to gain insight to likely causes of loss of
network connectivity. Not all of the commands listed are
available on some versions of Cisco operating systems. To
determine which commands are available for use with particular
devices, consult the online documentation for Cisco devices at
http://www.cisco.com/univercd/home/home.htm.
Content
3.4 Identifying Physical Layer Problems
3.4.1 Power related Power related issues are
the most fundamental reason for network failure. The main AC
power flows into either an external or internal AC to DC
transformer module within a device. The transformer provides
correctly modulated DC current, which acts to power device
circuits, connectors, ports and the blowers used for device
cooling. If a power related issue is suspected, a physical
inspection of the power module is often carried out. Initially,
with the power switch on, does the blower operate? - If
yes, the AC input checks out.
- If no, suspect the AC
input, AC source, router circuit breaker, or the power supply
cable.
- With the power switch on and system LEDs lit,
do the fans operate?
- If no, suspect the fans.
- Does the system shut down after being on a short time?
- Suspect an environmentally induced shutdown.
- Check the environmental site requirements in device
documentation and ensure that the chassis intake and exhaust
vents are clear.
- Suspect a power supply failure, have
other devices in the area powered down?
- System
partially boots, but LEDs do not light.
- Suspect a
5-volt (V) power supply failure.
Distribution and
core layer Cisco devices are generally modular in nature. This
approach can follow through for the power supply(s) of these
units. Often parallel and backup power supplies exist in the
chassis to balance the power load and to provide uninterrupted
operation in the event of failure of a single or multiple
online power modules. To help isolate a power subsystem
problem, follow these steps: - Check whether the power
supply LED labeled GOOD is on or the LED labeled FAIL is on.
- If the LED labeled GOOD is off or if the LED labeled
FAIL is on, take the following steps:
Step
1
Ensure that the power supply is flush with the back
of the chassis. Step 2
Unplug the power cord,
loosen and reinstall the power supply, tighten the captive
installation screws, and then plug in the power cord.
- If the LED labeled GOOD remains off, there might be a
problem with the AC source or the power cable. Connect the
power cord to another power source if one is available.
- If the LED labeled GOOD fails to light after the power
supply is connected to a new power source, replace the power
cord.
If this unit has more than one power cord,
repeat the steps for each power supply.
Content
3.4 Identifying Physical Layer Problems
3.4.2 Cabling faults – CAT5 Many problems can
be corrected by simply reseating cables that have become
partially disconnected. When performing a physical inspection,
look for damaged cables, improper cable types, and poorly
crimped RJ-45s. Suspect drop cables should be subject to a
simple cable test, and exchanged with a known-good cable. Do
not assume that just because a cable is new, just out of the
package, that it will work. Test it first. Cables made on site
should also be tested before attempting to troubleshoot with
them. Anyone can have a bad day and miswire the termination.
Also test for simple cable faults such as shorts, opens, and
split pairs. Determine whether a station connected by UTP or
fiber is successfully linking to the network at the MAC Layer.