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