Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...

166 CHAPTER 10 Network Troubleshooting
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Troubleshooting bridges and switches
■ Ensure to follow the 80/20 rule, which states that 80 percent of network
traffic should be local (occurring on the same side of the bridge or switch),
and not more than 20 percent should cross the bridge or switch.
■ Looping can occur when there is more than one active bridge or switch
on a network and the bridges and switches don’t know the location of a
destination computer.
■ Be sure to use the Spanning Tree Protocol when multiple bridges exist on
the network.
TROUBLESHOOTING THE NETWORK LAYER
When troubleshooting the network layer, you’ll be most concerned with routers
and TCP/IP addressing.
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Troubleshooting routers
■ Check for configuration errors or misconfiguration issues on each router
■ Check for a routing loop by using the tracert or traceroute command
■ Verify that a route exists to the destination network
■ Check for connectivity issues between the source and destination networks,
where either a router or a network link that’s required has failed
or gone offline.
Troubleshooting TCP/IP addressing
■ Use the ipconfig command to verify that the IP address, subnet
mask and default gateway, and other settings have been configured
correctly
■ Use the route command to verify that the default gateway and other
routing table entries are correct for an individual PC.
TROUBLESHOOTING THE TRANSPORT LAYER
When troubleshooting the transport layer, you’ll be most concerned with TCP and
UDP ports. You should be aware of the default TCP and UDP ports that are used by
major applications when you’re troubleshooting network issues at the transport
layer. Table 10.1 illustrates some of the more common TCP applications and the
ports they use, and Table 10.2 illustrates some of the more common UDP-based
applications and ports.
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Troubleshooting ports
■ Use the telnet command to see if a particular port is listening on the
destination machine.
■ Use the netstat utility to see a list of all ports that are listening on a
particular machine.