centrally located network analysis software or
even dedicated hardware. Simple Network Management Protocol
(SNMP) can be used to get each SNMP capable network device to
report back to a central monitoring host which is loaded with
software to interpret input from network devices. Similarly the
Network Analysis Module (NAM), designed for Cisco 6500 series
devices can become the nerve center for analysis of network
function. The NAM stores operational statistics collected using
remote VLAN and port centric or local VLAN Switchport Analyzer
sessions for each Cisco switching component of the network. The
data can then be compared to historical data. Increasing the
level of data capture occurring on network operational
statistics, a troubleshooter can gain valuable intelligence on
a network. This data can be used to isolate network problems
and also to identify under resourced or over utilized links and
components within the network.
Content 3.6
Implementing Physical Layer Solutions 3.6.1
Solving common problems – methodology A doctor would
not be considered successful unless he could follow up his
diagnosis of a patient’s problem with a successful solution.
The same idea applies to network troubleshooters. Isolating a
problem is a big step in troubleshooting, but the process is
not considered complete until the problem is solved and the
network returned to its baseline state. To correct an isolated
problem at the physical layer: - Make initial
configuration changes.
- If the correction requires
more than one change, make only one change at a time.
- Evaluate and document the results of each change
made.
- If the problem-solving steps are performed and
the results are unsuccessful, immediately undo the changes. If
the problem is intermittent, it may be necessary to wait to see
if the problem occurs again before evaluating the effect of any
changes.
- Stop making changes when the original
problem appears to be solved.
- Verify that the changes
made actually fixed the problem without introducing any new
problems.
- The network should be returned to the
baseline operation and no new or old symptoms should be
present. If the problem is not solved, undo all the changes
made. If new or additional problems are discovered during
trouble-shooting and problem correction, step back and modify
the correction plan.
- If necessary, get input from
outside resources.
- If none of the attempts to correct
the problem are successful, take the problem to another person.
This may be a coworker, consultant, or Cisco Technical
Assistance Center (TAC). On rare occasions it may be necessary
to perform a core dump, which creates output that a specialist
at Cisco Systems can analyze.
- Once the problem is
resolved, document the solution.
Content
3.6 Implementing Physical Layer
Solutions 3.6.2 ARP commands The address
resolution protocol is used by the IP network layer protocol to
map IP network addresses to the hardware addresses used by the
data-link protocol. The protocol operates below the network
layer as a part of the OSI data-link layer, and is used when IP
is used over the Internet. The MAC address is a data layer
address and is dependent on the interface card which is used.
IP operates at the network layer and is not concerned with IP
addresses of the nodes. A protocol known as the Address
Resolution Protocol (ARP) is therefore used to translate
between the two types of address. The ARP client and server
processes operate on all computers using IP over the Internet.
The processes are normally implemented as part of the software
driver which drives the NIC. An end-station will construct an
Address Resolution Table when it is connected to a network and
attempting to communicate with devices on its LAN segment. To
reduce the number of address resolution requests, a client
normally caches resolved addresses for a (short) period of
time. The ARP cache is of a finite size, and would become full
of incomplete and obsolete entries for computers that are not
in use if it was allowed to grow without check. The ARP cache
is therefore periodically flushed of all entries. This deletes
unused entries and frees space in the cache. It also removes
any unsuccessful attempts to contact hosts that are not
currently running. The arp command can be used on an
end-station to verify that the device is actually communicating
with other devices on its LAN segment. The presence of an ARP
table indicates that the end-station has at least been
connected to the network in the very recent past.
Content
3.6 Implementing Physical Layer
Solutions 3.6.3 Solving common problems –
Windows, UNIX/Mac OS end-system commands On Windows 2000
and XP systems the ipconfig command can be used to
establish if a NIC has successfully bound to the required IP
address. Sometimes a NIC will not release an old IP address.
Using the ipconfig /release command option forces the
NIC to release the currently held address. Following with the
ipconfig /renew command causes the NIC to attempt to
bind to an address supplied by a DHCP server, or to a manually
configured IP address. For UNIX and Mac OS X end-systems the
ifconfig –a command can be used to establish if a NIC
has bound to the correct IP address.
Content
3.6 Implementing Physical Layer
Solutions 3.6.4 Solving common problems – Cisco
IOS commands Interface configuration is a common source of
physical layer problems. If an interface configuration does not
match the corresponding configuration of a port on an attached
device then the interface status will be either:
- interface is down, line protocol is down
- interface
is up, line protocol is down
Commonly on serial
interfaces lack of physical connectivity could occur because
the interface at the clocking end has not been set with a clock
rate, or it has been set incorrectly. Also the no
shutdown command must be applied to the interface;
otherwise it will remain in an administratively down state,
which means that it has effectively been turned off. On serial
connections using Frame Relay or ISDN equivalent encapsulation,
protocols must be set at both sides of the link. HDLC has many
implementations, with different vendors creating different HDLC
versions that most often are proprietary and not
inter-compatible. Cisco has developed its own implementation of
HDLC. Amongst Cisco devices this is fine, however if a Cisco
device is connected via a serial link to another vendors
equipment it is usual to implement the IETF HDLC on both
devices. This version of HDLC is often used between dissimilar
equipment because it is a common second choice when a
proprietary version of HDLC will not suffice.
Content
3.6 Implementing Physical Layer
Solutions 3.6.5 Redundancy When installing
or maintaining a network it is important to have contingencies
in place to counter physical layer problems. The presence of an
Uninterruptible Power Supply (UPS), surge protectors, or line
filters can protect a network from short term power
interruptions, brownouts, and power spikes. Core layer hardware
should have replacement units on hand. It is a good idea to
install modular hardware wherever possible. In that way the
failure of a single module will not result in the entire device
being taken off line. Having replacement modules in store is
also cost effective. In the event of component failure, the
modules can be interchangeably used in a variety of compliant
chassis. When changes are made to device or network
configurations, back-up copies of the previous functional
configurations should be on hand in the event of network
failure resulting from these changes. In this way network
failure duration can be minimized and troubleshooting can occur
independent of the network. Having additional cabling and
testers on hand is also highly recommended. On network
installation it is important to protect backbone cabling from