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

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166 CHAPTER 10 Network Troubleshooting ■ 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. ■ ■ 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. ■ 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.
Troubleshooting the Session Layer 167 Table 10.1 Port Number Well-Known TCP Ports Application 20 FTP (data) 21 FTP (control) 22 Secure Shell (SSH) 23 Telnet 25 Simple Mail Transfer Protocol (SMTP) 53 DNS 80 Hyper Text Transfer Protocol (HTTP) 88 Kerberos <strong>11</strong>0 POP3 <strong>11</strong>9 Network News Transfer Protocol (NNTP) 139 NetBIOS 443 Secure sockets layer (SSL) Table 10.2 Port Number Well-Known UDP Ports Application 7 Echo 53 DNS query 69 Trivial File Transfer Protocol ( TFTP) 123 Network Time Protocol 161 SNMP TROUBLESHOOTING THE SESSION LAYER The functions of TCP/IP protocols from the session layer up to the application layer will often span all three layers, so it is not particularly common to perform troubleshooting that’s geared only toward the session layer of the OSI model. The most common issues you’ll see at the session layer involve slow network transmissions between two computers, which is caused by one computer in a connection using a half-duplex connection instead of a full-duplex connection.
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