Objective Area #3: Troubleshooting

For this objective area, Cisco expects you to know how to:

• Utilize the OSI model as a guide for systematic network troubleshooting.
• Perform LAN and VLAN troubleshooting.
• Troubleshoot routing protocols.
• Troubleshoot IP addressing and host configuration.
• Troubleshoot a device as part of a working network.
• Troubleshoot an access list.
• Perform simple WAN troubleshooting.

The OSI model is a no-brainer: How could any network guru possible get through an average day without knowing the OSI model? If you're not familiar with it yet, the OSI 7-layer reference model from the top is Application, Presentation, Session, Transport, Network, Data Link and Physical layers. The numbering starts from the bottom (physical layer is number 1). Knowing the layer names and their order is crucial. Some people like to use an anagram to help. One of my favorites is (from the bottom up) Please Do Not Throw Sausage Pizza Away.

Knowing where the most common network devices operate in the OSI model is also a must. At the physical layer there is the cabling of the network, repeaters, and hubs. The data link layer includes bridges and switches, and routers operate at the network layer.

Tip: A switch is a multi-port bridge.

As a CCNA, you need to understand protocol operation and at which layer each function. This exam focuses only on TCP/IP. From the physical layer up, there are protocols, such as V.35 and RS-232, that define the physical connection and signaling on the wire. At the data link layer protocols such as Ethernet, token-ring and FDDI are responsible for defining the network topology and data communication standards .

At the network layer, there is IP, ICMP ARP. At the transport layer, there is TCP and UDP. UDP is fast and efficient but does not provide guaranteed delivery and retransmission like TCP. TCP is more commonly used at this layer by many upper-layer protocols and applications. TCP uses windowing, buffering and acknowledgments for flow control. Skipping over the session and presentation layers for brevity, the application layer contains Telnet, FTP, TFTP, SMTP, and many other application-type protocols.

Troubleshooting IP addresses, subnet masks, and gateways requires knowledge of subnetting. You may be presented with a simple network diagram consisting of subnets, routers and hosts, and be asked to identify the incorrect assignment of IP addresses. You might also find a question relating to configuration of access lists. Remember the rules from above as well as the following: If you created an access list with various deny statements and no permit, you would prohibit the routing of all traffic on the applied interface. Remember there is the explicit deny at the end of each access list.

Tip: Ping, traceroute and show ip route can be used to troubleshoot connectivity problems.

Understanding how to perform the password recovery procedure on a Cisco router is must-have knowledge for the CCNA. Remember that on the 2500 series routers, the command config-register 0x2102 returns the bits from 0x2142, which were changed in the rommon mode using the command o/r for password recovery.