Remember the possible causes of LAN traffic congestion. Too many hosts in a broadcast domain, broadcast storms, multicasting, and low bandwidth are all possible causes of LAN traffic congestion.
Understand the difference between a collision domain and a broadcast domain. Collision domain is an Ethernet term used to describe a network collection of devices in which one particular device sends a packet on a network segment, forcing every other device on that same segment to pay attention to it. On a broadcast domain, a set of all devices on a network segment hear all broadcasts sent on that segment.
Understand the difference between a hub, a bridge, a switch, and a router. Hubs create one collision domain and one broadcast domain. Bridges break up collision domains but create one large broadcast domain. They use hardware addresses to filter the network. Switches are really just multiple port bridges with more intelligence. They break up collision domains but create one large broadcast domain by default. Switches use hardware addresses to filter the network. Routers break up broadcast domains (and collision domains) and use logical addressing to filter the network.
Remember the difference between connection-oriented and connectionless network services.
Connection-oriented services use acknowledgments and flow control to create a reliable session. More overhead is used than in a connectionless network service. Connectionless services are used to send data with no acknowledgments or flow control. This is considered unreliable.
Remember the OSI layers. You must remember the seven layers of the OSI model and what function each layer provides. The Application, Presentation, and Session layers are upper layers and are responsible for communicating from a user interface to an application. The Transport layer provides segmentation, sequencing, and virtual circuits. The Network layer provides logical network addressing and routing through an internetwork. The Data Link layer provides framing and placing of data on the network medium. The Physical layer is responsible for taking 1s and 0s and encoding them into a digital signal for transmission on the network segment.
Remember the types of Ethernet cabling and when you would use them. The three types of cables that can be created from an Ethernet cable are straight-through (to connect a PC’s or a router’s Ethernet interface to a hub or switch), crossover (to connect hub to hub, hub to switch, switch to switch, or PC to PC), and rolled (for a console connection from a PC to a router or switch).
Understand how to connect a console cable from a PC to a router and start HyperTerminal.
Take a rolled cable and connect it from the COM port of the host to the console port of a router. Start HyperTerminal and set the BPS to 9600 and flow control to None.
Remember the three layers in the Cisco three-layer model. The three layers in the Cisco hierarchical model are the core, distribution, and access layers.
Remember the various configuration register commands and settings. The 0x2102 setting is the default on all Cisco routers and tells the router to look in NVRAM for the boot sequence. 0x2101 tells the router to boot from ROM, and 0x2142 tells the router to not load the startupconfig in NVRAM to provide password recovery.
Remember how to back up an IOS image. By using the privileged-mode command copy flash tftp, you can back up a file from flash memory to a TFTP (network) server.
Remember how to restore or upgrade an IOS image. By using the privileged-mode command copy tftp flash, you can restore or upgrade a file from a TFTP (network) server to flash memory.
Remember what you must complete before you back up an IOS image to a network server.
Make sure that you can access the network server, ensure that the network server has adequate space for the code image, and verify the file naming and path requirement.
Remember how to save the configuration of a router. There are a couple of ways to do this, but the most common, as well as most tested, method is copy running-config startup-config.
Remember how to erase the configuration of a router. Type the privileged-mode command
erase startup-config and reload the router.
Understand when to use CDP. Cisco Discovery Protocol can be used to help you document as well as troubleshoot your network.
Remember what the output from the show cdp neighbors command shows. The show cdp neighbors command provides the following information: device ID, local interface, holdtime, capability, platform, and port ID (remote interface).
Understand how to telnet into a router and keep your connection but return to your originating console. If you telnet to a router or switch, you can end the connection by typing exit at any time. However, if you want to keep your connection to a remote device but still come back to your original router console, you can press the Ctrl+Shift+6 key combination, release it, and then press X.
Remember the command to verify your Telnet sessions. The command show sessions will provide you with information about all the sessions your router has with other routers.
Remember how to build a static host table on a router. By using the global configuration command
ip host host_name ip_address, you can build a static host table on your router. You can apply multiple IP addresses against the same host entry.
Remember how to verify your host table on a router. You can verify the host table with the show hosts command.
Remember the steps to subnet in your head. Understand how IP addressing and subnetting work. First, determine your block size by using the 256-subnet mask math. Then count your
subnets and determine the broadcast address of each subnet—it is always the number right before the next subnet. Your valid hosts are the numbers between the subnet address and the broadcast address.
Understand the various block sizes. This is an important part of understanding IP addressing and subnetting. The valid block sizes are always 4, 8, 16, 32, 64, 128, etc. You can determine your block size by using the 256-subnet mask math.
Remember the four diagnostic steps. The four simple steps that Cisco recommends for troubleshooting are ping the loopback address, ping the NIC, ping the default gateway, and ping the remote device.
You must be able to find and fix an IP addressing problem. Once you go through the four troubleshooting steps that Cisco recommends, you must be able to determine the IP addressing problem by drawing out the network and finding the valid and invalid hosts addressed in your network.
Understand the troubleshooting tools that you can use from your host and a Cisco router
ping 127.0.0.1 tests your local IP stack. tracert is a Windows DOS command to track the path a packet takes through an internetwork to a destination. Cisco routers use the command traceroute, or just trace for short. Don’t confuse the Windows and Cisco commands. Although they produce the same output, they don’t work from the same prompts. ipconfig /all will display your PC network configuration from a DOS prompt, and arp -a (again from a DOS prompt) will display IP-to-MAC-address mapping on a Windows PC.
