Information Technology Reference
In-Depth Information
General Switch Troubleshooting Suggestions
Many ways exist by which to troubleshoot a switch. As the features of switches grow, the possible things
that can break also increase. If you develop an approach or test plan for troubleshooting, you will be
better off in the long run than if you just try a hit-and-miss approach. Here are some general suggestions
for making your troubleshooting more effective:
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Take the time to become familiar with normal switch operation. Cisco's web site has a tremendous
amount of technical information describing how Cisco switches work, as mentioned in the previous
section. The configuration guides, in particular, are very helpful. Many cases opened with Cisco's
Technical Assistance Center (TAC) are solved with information from the product configuration
guides.
•
For the more complex situations, have an accurate physical and logical map of your network. A
physical map shows how the devices and cables are connected. A logical map shows what segments
(VLANs) exist in your network and which routers provide routing services to these segments. A
spanning-tree map is highly useful for troubleshooting complex issues. Because of a switch's
capability to create different segments by implementing VLANs, the physical connections alone do
not tell the whole story; you must know how the switches are configured to determine which
segments (VLANs) exist and to know how they are logically connected.
•
Have a plan. Some problems and solutions are obvious; some are not. The symptoms that you see
in your network may be the result of problems in another area or layer. Before jumping to
conclusions, try to verify in a structured way what is working and what is not. Because networks can
be complex, it is helpful to isolate possible problem domains. One way of doing this is by using the
OSI seven-layer model. For example, check the physical connections involved (Layer 1), check
connectivity issues within the VLAN (Layer 2), check connectivity issues across different VLANs
(Layer 3), and so on. Assuming a correct configuration on the switch, many of the problems that you
encounter will be related to physical layer issues (physical ports and cabling). Today, switches are
involved in Layer 3 and Layer 4 issues, incorporating intelligence to switch packets based on
information derived from routers, or by actually having routers living inside the switch (Layer 3 or
Layer 4 switching).
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Do not assume that a component is working without checking it first. This can save you a lot of
wasted time. For example, if a PC is not capable of logging into a server across your network, many
things could be wrong. Don't skip the basic things and assume that something works—someone
might have changed something without telling you. It takes only a minute to check some of the basic
things (for example, that the ports involved are connected to the right place and are active), which
could save you many wasted hours.
Troubleshooting Port Connectivity Problems
If the port doesn't work, nothing works! Ports are the foundation of your switching network. Some ports
have special significance because of their location in the network and the amount of traffic that they
carry. These ports would include connections to other switches, routers, and servers. These ports can be
more complicated to troubleshoot because they often take advantage of special features such as trunking
and EtherChannel. The rest of the ports are significant as well because they connect the actual users of
the network.
Many things can cause a port to be nonfunctional: hardware issues, configuration issues, and traffic
issues. Let's look at these categories a little deeper.
