One must understand how a network works before troubleshooting. A drawing of the network layout is a good way to begin. Each networked device (switches, hubs, routers as well as end devices) should be depicted on the drawing with information referring to data rates (10 or 100 Mbps), IP addresses and possibly MAC addresses. The drawing should illustrate the connections between all networked devices, and the type and length of cabling used in these connections.
Viewing network traffic is another way of gaining knowledge of the network. Hubs will allow a packet capture tool to view the network traffic. However, on Ethernet networks, hubs will also limit the physical wiring options of the network. Unmanaged Ethernet switches provide more wiring freedom; however, they will not allow a packet capture tool to view all the network traffic as switches isolate messages to the devices involved in the conversation. Managed Ethernet switches that provide a port mirroring capability allow a packet capture tool to view all network traffic.
Packet capture tools not only display normal working operation, they can also be helpful when diagnosing a problem. Ethereal (www.ethereal.com) is a free packet capture tool that also performs protocol analysis. Even if you do not fully understand the network protocol your equipment is using, Ethereal can provide some information concerning the communications for many protocols. Version 0.9.14 supports 512 protocols. It will display the packet in raw form as well as in a decoded form to help you decipher what is happening with your devices.
Most managed switches also provide SNMP as an aid in diagnosing problems. SNMP allows the user to view data collected in the switch for a number of operating parameters such as unicast messages sent and received on a port basis, number of multicast messages received and transmitted, number of broadcast messages, etc.
Ping is also a highly used network diagnostic tool. Ping allows someone to send and receive a series of ICMP (ping) messages between two Ethernet TCP/IP devices. TCP/IP compliant devices can use ping to confirm a connection between an end device and a computer executing ping.
The operating data rate and the duplex may also be important in diagnosing problems. Most switches support auto-negotiation of copper port data rate and duplex. Normally the switch and the end device will negotiate to the highest possible data rate and most favourable duplex supported by both devices. Issues in wiring and other problems may cause the network to operate at a lower than optimum level.
Many network problems are due to improper cabling. Using off-the-shelf copper cable can sometimes help save many hours of troubleshooting. If you wire your own cables, it is advisable to purchase an Ethernet cable tester. However, these devices can be fooled if the wrong wire pairs are used in the cable. One should look at the colour of wires used in each position of the RJ45 connector. The EIA/TIA 568A standard has two options for wire colours used in RJ-45 connectors. Our Ethernet slide chart has the EIA/TIA 568A wiring information along with other information which can assist in the troubleshooting of networks. This can be requested at www.ctrlink.com/reqform7.htm.
There are many tools and techniques available to help troubleshoot Ethernet networks. Different types of switches or hubs facilitate different troubleshooting techniques. One should consider this when purchasing network switches and hubs.
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